diff --git a/browser/base/content/browser.css b/browser/base/content/browser.css
index c9ebddb7f5c57..e00dd56ba2301 100644
--- a/browser/base/content/browser.css
+++ b/browser/base/content/browser.css
@@ -382,7 +382,7 @@ toolbarpaletteitem {
   toolbar[brighttext] & {
     list-style-image: var(--webextension-toolbar-image-light, inherit);
   }
-  toolbar:not([brighttext]) &:-moz-lwtheme {
+  :root[lwtheme] toolbar:not([brighttext]) {
     list-style-image: var(--webextension-toolbar-image-dark, inherit);
   }
   toolbaritem:is([overflowedItem="true"], [cui-areatype="panel"]) > & {
diff --git a/browser/themes/linux/browser.css b/browser/themes/linux/browser.css
index 89df26a2f07d0..814a2164ea6ae 100644
--- a/browser/themes/linux/browser.css
+++ b/browser/themes/linux/browser.css
@@ -16,7 +16,7 @@
  * disabling the toolbar field border and backgrounds.
  */
 @media not (prefers-contrast) {
-  :root:not(:-moz-lwtheme) {
+  :root:not([lwtheme]) {
     --toolbar-field-border-color: transparent;
 
     /* These colors don't exactly match the default dark color that buttons and
@@ -237,7 +237,7 @@
   @media (-moz-bool-pref: "widget.gtk.non-native-titlebar-buttons.enabled") {
     /* When using lightweight themes, use our own buttons since native ones might
      * assume a native background in order to be visible. */
-    &:-moz-lwtheme {
+    :root[lwtheme] & {
       padding-inline: 3px;
 
       > .toolbarbutton-icon {
diff --git a/browser/themes/osx/browser.css b/browser/themes/osx/browser.css
index b62710cc71ebb..b4c10258a8ff3 100644
--- a/browser/themes/osx/browser.css
+++ b/browser/themes/osx/browser.css
@@ -231,7 +231,7 @@ moz-input-box > menupopup .context-menu-add-engine > .menu-iconic-left {
     text-shadow: inherit;
 
     @media (prefers-color-scheme: light) {
-      &:not(:-moz-lwtheme) {
+      :root:not([lwtheme]) & {
         /* overriding tabbox.css */
         color: hsl(240, 5%, 5%);
       }
diff --git a/browser/themes/shared/addons/unified-extensions.css b/browser/themes/shared/addons/unified-extensions.css
index fd671e007d1d0..baf0697efdf6b 100644
--- a/browser/themes/shared/addons/unified-extensions.css
+++ b/browser/themes/shared/addons/unified-extensions.css
@@ -179,56 +179,52 @@ unified-extensions-item > .subviewbutton {
 /* --- browser action CUI widget styles in the extensions panel --- */
 
 @media (prefers-contrast) {
-  :root:not(:-moz-lwtheme) {
+  :root:not([lwtheme]) {
     --uei-button-attention-dot-color: ButtonText;
-  }
-
-  .unified-extensions-item-action-button.subviewbutton:not([disabled], :-moz-lwtheme),
-  .unified-extensions-item-menu-button.subviewbutton > .toolbarbutton-icon:not(:-moz-lwtheme) {
-    border-color: currentColor;
-    background-color: ButtonFace;
-    color: ButtonText;
-    --uei-button-hover-bgcolor: SelectedItem;
-    --uei-button-hover-color: SelectedItemText;
-    --uei-button-active-bgcolor: ButtonFace;
-    --uei-button-active-color: ButtonText;
-  }
-
-  .unified-extensions-item-action-button[disabled].subviewbutton:not(:-moz-lwtheme) {
-    background-color: Canvas;
-    color: GrayText !important; /* override panelUI-shared.css */
-    opacity: 1 !important; /* override panelUI-shared.css */
-  }
-
-  .unified-extensions-item[attention] > .unified-extensions-item-action-button.subviewbutton:hover:not(:-moz-lwtheme) {
-    --uei-button-attention-dot-color: ButtonFace;
-  }
-
-  .unified-extensions-item[attention] > .unified-extensions-item-action-button.subviewbutton:hover:active:not(:-moz-lwtheme) {
-    --uei-button-attention-dot-color: ButtonText;
-  }
-
-  .unified-extensions-item-message:not(:-moz-lwtheme) {
-    color: inherit;
-  }
-
-  .unified-extensions-item > .unified-extensions-item-action-button.subviewbutton:hover:not([disabled], :-moz-lwtheme),
-  .unified-extensions-item > .unified-extensions-item-menu-button.subviewbutton:hover > .toolbarbutton-icon:not(:-moz-lwtheme) {
-    background-color: var(--uei-button-hover-bgcolor);
-    color: var(--uei-button-hover-color);
-    border-color: var(--uei-button-hover-bgcolor);
-  }
-
-  .unified-extensions-item > .unified-extensions-item-action-button.subviewbutton:hover:active:not([disabled], :-moz-lwtheme),
-  .unified-extensions-item > .unified-extensions-item-menu-button.subviewbutton:hover:active > .toolbarbutton-icon:not(:-moz-lwtheme) {
-    background-color: var(--uei-button-active-bgcolor);
-    color: var(--uei-button-active-color);
-    border-color: var(--uei-button-active-color);
-  }
 
-  .unified-extensions-item > .unified-extensions-item-menu-button.subviewbutton:focus-visible > .toolbarbutton-icon:not(:-moz-lwtheme) {
-    /* The border would otherwise overlap with the focus outline, causing an
-     * unsightly anti-aliasing artifact */
-    border-color: transparent;
+    .unified-extensions-item-action-button.subviewbutton:not([disabled]),
+    .unified-extensions-item-menu-button.subviewbutton > .toolbarbutton-icon {
+      border-color: currentColor;
+      background-color: ButtonFace;
+      color: ButtonText;
+      --uei-button-hover-bgcolor: SelectedItem;
+      --uei-button-hover-color: SelectedItemText;
+      --uei-button-active-bgcolor: ButtonFace;
+      --uei-button-active-color: ButtonText;
+    }
+
+    .unified-extensions-item-action-button[disabled].subviewbutton {
+      background-color: Canvas;
+      color: GrayText !important; /* override panelUI-shared.css */
+      opacity: 1 !important; /* override panelUI-shared.css */
+    }
+
+    .unified-extensions-item[attention] > .unified-extensions-item-action-button.subviewbutton:hover:not(:active) {
+      --uei-button-attention-dot-color: ButtonFace;
+    }
+
+    .unified-extensions-item-message {
+      color: inherit;
+    }
+
+    .unified-extensions-item > .unified-extensions-item-action-button.subviewbutton:hover:not([disabled]),
+    .unified-extensions-item > .unified-extensions-item-menu-button.subviewbutton:hover > .toolbarbutton-icon {
+      background-color: var(--uei-button-hover-bgcolor);
+      color: var(--uei-button-hover-color);
+      border-color: var(--uei-button-hover-bgcolor);
+    }
+
+    .unified-extensions-item > .unified-extensions-item-action-button.subviewbutton:hover:active:not([disabled]),
+    .unified-extensions-item > .unified-extensions-item-menu-button.subviewbutton:hover:active > .toolbarbutton-icon {
+      background-color: var(--uei-button-active-bgcolor);
+      color: var(--uei-button-active-color);
+      border-color: var(--uei-button-active-color);
+    }
+
+    .unified-extensions-item > .unified-extensions-item-menu-button.subviewbutton:focus-visible > .toolbarbutton-icon {
+      /* The border would otherwise overlap with the focus outline, causing an
+       * unsightly anti-aliasing artifact */
+      border-color: transparent;
+    }
   }
 }
diff --git a/browser/themes/shared/browser-custom-colors.css b/browser/themes/shared/browser-custom-colors.css
index d98d56690f8e0..7d689e019e1fc 100644
--- a/browser/themes/shared/browser-custom-colors.css
+++ b/browser/themes/shared/browser-custom-colors.css
@@ -5,7 +5,7 @@
 @namespace html url("http://www.w3.org/1999/xhtml");
 
 @media not (prefers-contrast) {
-  :root:not(:-moz-lwtheme) {
+  :root:not([lwtheme]) {
     --button-primary-bgcolor: light-dark(rgb(0, 97, 224), rgb(0, 221, 255));
     --button-primary-hover-bgcolor: light-dark(rgb(2, 80, 187), rgb(128, 235, 255));
     --button-primary-active-bgcolor: light-dark(rgb(5, 62, 148), rgb(170, 242, 255));
diff --git a/browser/themes/shared/browser-shared.css b/browser/themes/shared/browser-shared.css
index f708347193241..6fd9c96560429 100644
--- a/browser/themes/shared/browser-shared.css
+++ b/browser/themes/shared/browser-shared.css
@@ -104,7 +104,7 @@
     --toolbox-non-lwt-textcolor-inactive: InactiveCaptionText;
   }
 
-  &:-moz-lwtheme {
+  &[lwtheme] {
     color: var(--lwt-text-color);
 
     --link-color: light-dark(rgb(0, 97, 224), rgb(0, 221, 255));
@@ -170,7 +170,7 @@
     border-bottom-style: none;
   }
 
-  &:-moz-lwtheme {
+  :root[lwtheme] & {
     background-image: var(--lwt-additional-images);
     background-repeat: var(--lwt-background-tiling);
     background-position: var(--lwt-background-alignment);
@@ -218,7 +218,7 @@
   padding-inline-start: var(--toolbar-start-end-padding);
 }
 
-:root[sessionrestored] #nav-bar:-moz-lwtheme {
+:root[sessionrestored][lwtheme] #nav-bar {
   transition: var(--ext-theme-background-transition);
 }
 
@@ -480,22 +480,22 @@ menupopup::part(drop-indicator) {
   color-scheme: var(--toolbar-color-scheme);
   border-top-color: var(--chrome-content-separator-color, ThreeDShadow);
 
-  &:-moz-lwtheme {
+  :root[lwtheme] & {
     background-color: var(--lwt-accent-color);
     background-image: linear-gradient(var(--toolbar-bgcolor), var(--toolbar-bgcolor)), var(--lwt-additional-images);
     background-repeat: no-repeat, var(--lwt-background-tiling);
     background-position: right bottom, var(--lwt-background-alignment);
     background-position-y: bottom !important;
+  }
 
-    :root:not([lwtheme-image]) &:-moz-window-inactive {
-      background-color: var(--lwt-accent-color-inactive, var(--lwt-accent-color));
-    }
+  :root[lwtheme]:not([lwtheme-image]) &:-moz-window-inactive {
+    background-color: var(--lwt-accent-color-inactive, var(--lwt-accent-color));
+  }
 
-    :root[lwtheme-image] & {
-      background-image: linear-gradient(var(--toolbar-bgcolor), var(--toolbar-bgcolor)), var(--lwt-header-image), var(--lwt-additional-images);
-      background-repeat: no-repeat, no-repeat, var(--lwt-background-tiling);
-      background-position: center, right bottom, var(--lwt-background-alignment);
-    }
+  :root[lwtheme-image] & {
+    background-image: linear-gradient(var(--toolbar-bgcolor), var(--toolbar-bgcolor)), var(--lwt-header-image), var(--lwt-additional-images);
+    background-repeat: no-repeat, no-repeat, var(--lwt-background-tiling);
+    background-position: center, right bottom, var(--lwt-background-alignment);
   }
 }
 
diff --git a/browser/themes/shared/customizableui/customizeMode.css b/browser/themes/shared/customizableui/customizeMode.css
index b3ee1ada6cbae..a1bb6a62f97a3 100644
--- a/browser/themes/shared/customizableui/customizeMode.css
+++ b/browser/themes/shared/customizableui/customizeMode.css
@@ -24,18 +24,18 @@
   background-color: var(--toolbar-non-lwt-bgcolor);
   color: var(--toolbar-color);
   color-scheme: var(--toolbar-color-scheme);
-}
 
-#customization-container:-moz-lwtheme {
-  /* Ensure this displays on top of the non-lwt bgcolor, to make sure it's not
-   * semi-transparent */
-  background-image: linear-gradient(var(--toolbar-bgcolor), var(--toolbar-bgcolor));
-}
+  :root[lwtheme] & {
+    /* Ensure this displays on top of the non-lwt bgcolor, to make sure it's not
+     * semi-transparent */
+    background-image: linear-gradient(var(--toolbar-bgcolor), var(--toolbar-bgcolor));
+  }
 
-:root[lwtheme-image] #customization-container {
-  background-image: none;
-  color: var(--toolbar-non-lwt-textcolor);
-  text-shadow: none;
+  :root[lwtheme-image] & {
+    background-image: none;
+    color: var(--toolbar-non-lwt-textcolor);
+    text-shadow: none;
+  }
 }
 
 :root[lwtheme-image] #customization-palette .toolbarbutton-1 {
diff --git a/browser/themes/shared/customizableui/panelUI-shared.css b/browser/themes/shared/customizableui/panelUI-shared.css
index 51ab66b25ab24..f3647f12dabe4 100644
--- a/browser/themes/shared/customizableui/panelUI-shared.css
+++ b/browser/themes/shared/customizableui/panelUI-shared.css
@@ -33,7 +33,7 @@
 
   --panel-and-palette-icon-size: 16px;
 
-  &:not(:-moz-lwtheme) {
+  &:not([lwtheme]) {
     --panel-separator-zap-gradient: linear-gradient(90deg, #9059FF 0%, #FF4AA2 52.08%, #FFBD4F 100%);
   }
 
diff --git a/browser/themes/shared/downloads/progressmeter.css b/browser/themes/shared/downloads/progressmeter.css
index 3184a63ed539c..ad9f5f5f7dc9d 100644
--- a/browser/themes/shared/downloads/progressmeter.css
+++ b/browser/themes/shared/downloads/progressmeter.css
@@ -8,12 +8,13 @@
   --download-progress-fill-color: AccentColor;
   --download-progress-paused-color: GrayText;
   --download-progress-flare-color: rgba(255,255,255,0.75);
-}
-
-/* download progress bar is used in contexts which are not LightweightThemeConsumers and
-   do not get the lwt- theme variables. So we duplicate the fallbacks here. */
-:root:-moz-lwtheme {
-  --download-progress-fill-color: var(--lwt-toolbarbutton-icon-fill-attention, light-dark(rgb(0, 97, 224), rgb(0, 221, 255)));
+  &[lwtheme] {
+    /* download progress bar is used in contexts which are not LightweightThemeConsumers and
+       do not get the lwt- theme variables. So we duplicate the fallbacks here.
+       FIXME(emilio): But then how do we get the lwtheme attribute? I think the
+       above makes no sense */
+    --download-progress-fill-color: var(--lwt-toolbarbutton-icon-fill-attention, light-dark(rgb(0, 97, 224), rgb(0, 221, 255)));
+  }
 }
 
 @media (prefers-color-scheme: dark) {
diff --git a/browser/themes/shared/tabs.css b/browser/themes/shared/tabs.css
index eb92f71e5942a..36eb1c68b3db5 100644
--- a/browser/themes/shared/tabs.css
+++ b/browser/themes/shared/tabs.css
@@ -19,7 +19,7 @@
   --tab-block-margin: 4px;
 
   --tab-attention-icon-color: AccentColor;
-  &:-moz-lwtheme {
+  &[lwtheme] {
     --tab-attention-icon-color: light-dark(rgb(42, 195, 162), rgb(84, 255, 189));
   }
 
@@ -474,8 +474,8 @@
 }
 
 @media not (prefers-contrast) {
-  #TabsToolbar #firefox-view-button[open]:not(:focus-visible) > .toolbarbutton-icon:-moz-lwtheme,
-  .tab-background[selected]:not([multiselected]):-moz-lwtheme {
+  :root[lwtheme] #TabsToolbar #firefox-view-button[open]:not(:focus-visible) > .toolbarbutton-icon,
+  :root[lwtheme] .tab-background[selected]:not([multiselected]) {
     outline: 1px solid var(--lwt-tab-line-color, var(--lwt-tabs-border-color, currentColor));
     outline-offset: -1px;
   }
diff --git a/browser/themes/shared/toolbarbutton-icons.css b/browser/themes/shared/toolbarbutton-icons.css
index 3879689f12542..c10a9db2cca01 100644
--- a/browser/themes/shared/toolbarbutton-icons.css
+++ b/browser/themes/shared/toolbarbutton-icons.css
@@ -6,14 +6,13 @@
   --toolbarbutton-icon-fill-attention: AccentColor;
   --toolbarbutton-icon-fill-attention-text: AccentColorText;
   --warning-icon-bgcolor: light-dark(#FFA436, #FFBD4F);
-}
-
-:root:-moz-lwtheme {
-  --toolbarbutton-icon-fill-attention: var(--lwt-toolbarbutton-icon-fill-attention, light-dark(rgb(0, 97, 224), rgb(0, 221, 255)));
-  /* FIXME(emilio): This could have poor contrast on some themes, maybe use
-   * contrast-color() once that's available, or compute a new variable in
-   * LightWeightThemeConsumer */
-  --toolbarbutton-icon-fill-attention-text: var(--toolbar-field-background-color);
+  &[lwtheme] {
+    --toolbarbutton-icon-fill-attention: var(--lwt-toolbarbutton-icon-fill-attention, light-dark(rgb(0, 97, 224), rgb(0, 221, 255)));
+    /* FIXME(emilio): This could have poor contrast on some themes, maybe use
+     * contrast-color() once that's available, or compute a new variable in
+     * LightWeightThemeConsumer */
+    --toolbarbutton-icon-fill-attention-text: var(--toolbar-field-background-color);
+  }
 }
 
 .toolbarbutton-animatable-box,
diff --git a/browser/themes/shared/urlbar-searchbar.css b/browser/themes/shared/urlbar-searchbar.css
index 9fc88fbde80a3..5d35daecc9777 100644
--- a/browser/themes/shared/urlbar-searchbar.css
+++ b/browser/themes/shared/urlbar-searchbar.css
@@ -126,15 +126,17 @@
   border-color: color-mix(in srgb, currentColor 20%, transparent);
 }
 
-#urlbar-input:-moz-lwtheme::selection,
-.searchbar-textbox:-moz-lwtheme::selection {
-  background-color: var(--lwt-toolbar-field-highlight, Highlight);
-  color: var(--lwt-toolbar-field-highlight-text, HighlightText);
-}
+:root[lwtheme] {
+  #urlbar-input::selection,
+  .searchbar-textbox::selection {
+    background-color: var(--lwt-toolbar-field-highlight, Highlight);
+    color: var(--lwt-toolbar-field-highlight-text, HighlightText);
+  }
 
-#urlbar-input:not(:focus):-moz-lwtheme::selection,
-.searchbar-textbox:not(:focus-within):-moz-lwtheme::selection {
-  background-color: var(--lwt-toolbar-field-highlight, text-select-disabled-background);
+  #urlbar-input:not(:focus)::selection,
+  .searchbar-textbox:not(:focus-within)::selection {
+    background-color: var(--lwt-toolbar-field-highlight, text-select-disabled-background);
+  }
 }
 
 #urlbar:not([focused="true"]) {
@@ -376,7 +378,7 @@
 
   /* As above, but for the default theme in dark mode, which suffers from the same issue */
   @media (prefers-color-scheme: dark) {
-    &:not(:-moz-lwtheme) {
+    :root:not([lwtheme]) & {
       filter: grayscale(100%) brightness(20%) invert();
     }
   }
@@ -791,7 +793,7 @@
   margin: var(--arrowpanel-menuitem-margin);
   width: auto;
 
-  & > #searchbar:-moz-lwtheme {
+  :root[lwtheme] & > #searchbar {
     /* Theme authors usually only consider contrast against the toolbar when
        picking a border color for the search bar. Since the search bar can be
        dragged into the overflow panel, we need to show a high-contrast border
diff --git a/browser/themes/shared/urlbarView.css b/browser/themes/shared/urlbarView.css
index ee8ee15c2aa41..756742b70854a 100644
--- a/browser/themes/shared/urlbarView.css
+++ b/browser/themes/shared/urlbarView.css
@@ -56,7 +56,7 @@
     --urlbarView-action-slide-in-distance: -200px;
   }
 
-  &:-moz-lwtheme {
+  &[lwtheme] {
     --urlbarView-action-color: light-dark(rgb(91, 91, 102), rgb(191, 191, 201));
     --urlbarView-highlight-background: light-dark(rgb(0, 99, 255), rgb(0, 99, 225));
     --urlbarView-highlight-color: white;
diff --git a/browser/themes/windows/browser.css b/browser/themes/windows/browser.css
index c4c2f814c76d4..92df590982af3 100644
--- a/browser/themes/windows/browser.css
+++ b/browser/themes/windows/browser.css
@@ -35,7 +35,7 @@
       --toolbox-non-lwt-bgcolor-inactive: InactiveCaption;
       --toolbox-non-lwt-textcolor-inactive: InactiveCaptionText;
 
-      #TabsToolbar:not(:-moz-lwtheme) {
+      &:not([lwtheme]) #TabsToolbar {
         /* These colors match the Linux/HCM default button colors. We need to
          * override these on the tabs toolbar because the accent color is
          * arbitrary, so the hardcoded colors from browser-custom-colors might
@@ -309,10 +309,12 @@
     --urlbar-box-hover-text-color: SelectedItemText;
   }
 
-  #urlbar:not(:-moz-lwtheme, [focused="true"]) > #urlbar-background,
-  #searchbar:not(:-moz-lwtheme, :focus-within),
-  .findbar-textbox:not(:-moz-lwtheme, :focus) {
-    border-color: ThreeDShadow;
+  :root:not([lwtheme]) {
+    #urlbar:not([focused="true"]) > #urlbar-background,
+    #searchbar:not(:focus-within),
+    .findbar-textbox:not(:focus) {
+      border-color: ThreeDShadow;
+    }
   }
 }
 
diff --git a/dom/base/Document.cpp b/dom/base/Document.cpp
index 4e9286a91e3b0..918fbb497db7b 100644
--- a/dom/base/Document.cpp
+++ b/dom/base/Document.cpp
@@ -12537,14 +12537,6 @@ void Document::UpdateDocumentStates(DocumentState aMaybeChangedStates,
     }
   }
 
-  if (aMaybeChangedStates.HasAtLeastOneOfStates(DocumentState::LWTHEME)) {
-    if (ComputeDocumentLWTheme()) {
-      mState |= DocumentState::LWTHEME;
-    } else {
-      mState &= ~DocumentState::LWTHEME;
-    }
-  }
-
   if (aMaybeChangedStates.HasState(DocumentState::WINDOW_INACTIVE)) {
     BrowsingContext* bc = GetBrowsingContext();
     if (!bc || !bc->GetIsActiveBrowserWindow()) {
@@ -16558,16 +16550,6 @@ void Document::SetStateObject(nsIStructuredCloneContainer* scContainer) {
   mCachedStateObjectValid = false;
 }
 
-bool Document::ComputeDocumentLWTheme() const {
-  if (!NodePrincipal()->IsSystemPrincipal()) {
-    return false;
-  }
-
-  Element* element = GetRootElement();
-  return element && element->AttrValueIs(kNameSpaceID_None, nsGkAtoms::lwtheme,
-                                         nsGkAtoms::_true, eCaseMatters);
-}
-
 already_AddRefed<Element> Document::CreateHTMLElement(nsAtom* aTag) {
   RefPtr<mozilla::dom::NodeInfo> nodeInfo;
   nodeInfo = mNodeInfoManager->GetNodeInfo(aTag, nullptr, kNameSpaceID_XHTML,
diff --git a/dom/base/Document.h b/dom/base/Document.h
index a52c61addffc4..59f4c685b1cd4 100644
--- a/dom/base/Document.h
+++ b/dom/base/Document.h
@@ -2992,15 +2992,6 @@ class Document : public nsINode,
     SetStateObject(aDocument->mStateObjectContainer);
   }
 
-  /**
-   * Returns true if there is a lightweight theme specified. This is used to
-   * determine the state of the :-moz-lwtheme pseudo-class.
-   */
-  bool ComputeDocumentLWTheme() const;
-  void ResetDocumentLWTheme() {
-    UpdateDocumentStates(DocumentState::LWTHEME, true);
-  }
-
   // Whether we're a media document or not.
   enum class MediaDocumentKind {
     NotMedia,
diff --git a/dom/base/rust/lib.rs b/dom/base/rust/lib.rs
index 1c155e062b18b..467eeeb832180 100644
--- a/dom/base/rust/lib.rs
+++ b/dom/base/rust/lib.rs
@@ -156,8 +156,6 @@ bitflags! {
         const RTL_LOCALE = 1 << 1;
         /// LTR locale: specific to the XUL localedir attribute
         const LTR_LOCALE = 1 << 2;
-        /// LWTheme status
-        const LWTHEME = 1 << 3;
 
         const ALL_LOCALEDIR_BITS = Self::LTR_LOCALE.bits() | Self::RTL_LOCALE.bits();
     }
diff --git a/dom/xul/ChromeObserver.cpp b/dom/xul/ChromeObserver.cpp
index 9ad66348d41ba..73f712e30cc8e 100644
--- a/dom/xul/ChromeObserver.cpp
+++ b/dom/xul/ChromeObserver.cpp
@@ -137,10 +137,6 @@ void ChromeObserver::AttributeChanged(dom::Element* aElement,
       // if the localedir changed on the root element, reset the document
       // direction
       mDocument->ResetDocumentDirection();
-    } else if (aName == nsGkAtoms::lwtheme) {
-      // if the lwtheme changed, make sure to reset the document lwtheme
-      // cache
-      mDocument->ResetDocumentLWTheme();
     }
   } else {
     if (aName == nsGkAtoms::hidechrome) {
@@ -151,9 +147,6 @@ void ChromeObserver::AttributeChanged(dom::Element* aElement,
       // if the localedir changed on the root element, reset the document
       // direction
       mDocument->ResetDocumentDirection();
-    } else if (aName == nsGkAtoms::lwtheme) {
-      // if the lwtheme changed, make sure to restyle appropriately
-      mDocument->ResetDocumentLWTheme();
     } else if (aName == nsGkAtoms::drawtitle) {
       SetDrawsTitle(false);
     }
diff --git a/layout/style/test/test_non_content_accessible_pseudos.html b/layout/style/test/test_non_content_accessible_pseudos.html
index 81493b1a4af7e..5f2d15d6a25db 100644
--- a/layout/style/test/test_non_content_accessible_pseudos.html
+++ b/layout/style/test/test_non_content_accessible_pseudos.html
@@ -30,7 +30,6 @@
   ":-moz-dir-attr-rtl",
   ":-moz-dir-attr-like-auto",
   ":-moz-autofill-preview",
-  ":-moz-lwtheme",
   ":-moz-is-html",
   ":-moz-locale-dir(rtl)",
   ":-moz-locale-dir(ltr)",
diff --git a/layout/style/test/test_selectors.html b/layout/style/test/test_selectors.html
index d688139d3c513..684e9d6524d60 100644
--- a/layout/style/test/test_selectors.html
+++ b/layout/style/test/test_selectors.html
@@ -1077,7 +1077,6 @@
     test_balanced_unparseable(":dir");
 
     // Test chrome-only -moz-lwtheme
-    test_balanced_unparseable(":-moz-lwtheme");
     test_balanced_unparseable(":-moz-broken");
 
     test_balanced_unparseable(":-moz-tree-row(selected)");
diff --git a/servo/components/style/gecko/non_ts_pseudo_class_list.rs b/servo/components/style/gecko/non_ts_pseudo_class_list.rs
index cc7495dd9c086..f628c77ad51fb 100644
--- a/servo/components/style/gecko/non_ts_pseudo_class_list.rs
+++ b/servo/components/style/gecko/non_ts_pseudo_class_list.rs
@@ -98,7 +98,6 @@ macro_rules! apply_non_ts_list {
                 // media query results are more expensive than document state changes. So for now
                 // making them pseudo-classes is probably the right trade-off.
                 ("-moz-is-html", MozIsHTML, _, PSEUDO_CLASS_ENABLED_IN_UA_SHEETS),
-                ("-moz-lwtheme", MozLWTheme, _, PSEUDO_CLASS_ENABLED_IN_UA_SHEETS_AND_CHROME),
                 ("-moz-window-inactive", MozWindowInactive, _, _),
             ]
         }
diff --git a/servo/components/style/gecko/selector_parser.rs b/servo/components/style/gecko/selector_parser.rs
index 203e6a3609f58..100bb45e81602 100644
--- a/servo/components/style/gecko/selector_parser.rs
+++ b/servo/components/style/gecko/selector_parser.rs
@@ -196,7 +196,6 @@ impl NonTSPseudoClass {
                 None => DocumentState::empty(),
             },
             NonTSPseudoClass::MozWindowInactive => DocumentState::WINDOW_INACTIVE,
-            NonTSPseudoClass::MozLWTheme => DocumentState::LWTHEME,
             _ => DocumentState::empty(),
         }
     }
@@ -214,11 +213,10 @@ impl NonTSPseudoClass {
                       NonTSPseudoClass::MozNativeAnonymous |
                       // :-moz-placeholder is parsed but never matches.
                       NonTSPseudoClass::MozPlaceholder |
-                      // :-moz-is-html, :-moz-lwtheme, :-moz-locale-dir and :-moz-window-inactive
+                      // :-moz-is-html, :-moz-locale-dir and :-moz-window-inactive
                       // depend only on the state of the document, which is invariant across all
                       // elements involved in a given style cache.
                       NonTSPseudoClass::MozIsHTML |
-                      NonTSPseudoClass::MozLWTheme |
                       NonTSPseudoClass::MozLocaleDir(_) |
                       NonTSPseudoClass::MozWindowInactive
             )
diff --git a/servo/components/style/gecko/wrapper.rs b/servo/components/style/gecko/wrapper.rs
index 43cf6e7941894..ade352737c384 100644
--- a/servo/components/style/gecko/wrapper.rs
+++ b/servo/components/style/gecko/wrapper.rs
@@ -2101,7 +2101,6 @@ impl<'le> ::selectors::Element for GeckoElement<'le> {
                 bindings::Gecko_IsSelectListBox(self.0)
             },
             NonTSPseudoClass::MozIsHTML => self.as_node().owner_doc().is_html_document(),
-            NonTSPseudoClass::MozLWTheme |
             NonTSPseudoClass::MozLocaleDir(..) |
             NonTSPseudoClass::MozWindowInactive => {
                 let state_bit = pseudo_class.document_state_flag();
diff --git a/toolkit/themes/osx/global/toolbar.css b/toolkit/themes/osx/global/toolbar.css
index 0a60ae57d9261..24a0c58739e08 100644
--- a/toolkit/themes/osx/global/toolbar.css
+++ b/toolkit/themes/osx/global/toolbar.css
@@ -18,7 +18,7 @@ toolbar {
   -moz-default-appearance: toolbar;
 }
 
-toolbar:-moz-lwtheme {
+*|*:root[lwtheme] toolbar {
   appearance: none;
 }
 
diff --git a/toolkit/themes/shared/findbar.css b/toolkit/themes/shared/findbar.css
index 714324929e42b..a3c2ca09b7d90 100644
--- a/toolkit/themes/shared/findbar.css
+++ b/toolkit/themes/shared/findbar.css
@@ -2,10 +2,10 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
-@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
+@namespace xul url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul");
 @namespace html url("http://www.w3.org/1999/xhtml");
 
-findbar {
+xul|findbar {
   border-top: 1px solid ThreeDShadow;
   min-width: 1px;
   transition-property: margin-bottom, visibility;
@@ -14,29 +14,31 @@ findbar {
   padding-block: 6px;
   background-color: -moz-dialog;
   color: -moz-dialogtext;
-}
 
-findbar[hidden] {
-  /* Override display:none to make the transition work. */
-  display: flex;
-  margin-bottom: calc(-1 * (28px + 12px + 1px)); /* findbar-container's height + padding-block + top border */
-  visibility: collapse;
-  transition-delay: 0s, 150ms;
-}
+  &:where([hidden]) {
+    /* Override display:none to make the transition work. */
+    display: flex;
+    margin-bottom: calc(-1 * (28px + 12px + 1px)); /* findbar-container's height + padding-block + top border */
+    visibility: collapse;
+    transition-delay: 0s, 150ms;
+
+    > .findbar-container,
+    > .findbar-closebutton {
+      opacity: 0;
+    }
+  }
 
-@media (prefers-reduced-motion: reduce) {
-  findbar,
-  findbar[hidden] {
+  @media (prefers-reduced-motion) {
     transition-duration: 0s;
     transition-delay: 0s;
   }
-}
 
-findbar[noanim],
-findbar[noanim] > .findbar-container,
-findbar[noanim] > .findbar-closebutton {
-  transition-duration: 0s;
-  transition-delay: 0s;
+  &[noanim],
+  &[noanim] > .findbar-container,
+  &[noanim] > .findbar-closebutton {
+    transition-duration: 0s;
+    transition-delay: 0s;
+  }
 }
 
 .findbar-container {
@@ -46,10 +48,6 @@ findbar[noanim] > .findbar-closebutton {
   transition: opacity 150ms ease-in-out;
 }
 
-findbar[hidden] > .findbar-container {
-  opacity: 0;
-}
-
 /* Remove start margin when close button is on the left side (on macOS) */
 .findbar-closebutton + .findbar-container {
   margin-inline-start: 0;
@@ -57,7 +55,7 @@ findbar[hidden] > .findbar-container {
 
 /* Search field */
 
-html|input.findbar-textbox {
+.findbar-textbox {
   appearance: none;
   background-color: var(--input-bgcolor, var(--toolbar-field-background-color));
   color: var(--input-color, var(--toolbar-field-color));
@@ -69,39 +67,39 @@ html|input.findbar-textbox {
   padding-inline-start: 8px;
   width: 18em;
   box-sizing: border-box;
-}
 
-html|input.findbar-textbox::placeholder {
-  opacity: 0.69;
-}
+  &::placeholder {
+    opacity: 0.69;
+  }
 
-html|input.findbar-textbox:focus {
-  background-color: var(--toolbar-field-focus-background-color);
-  color: var(--toolbar-field-focus-color);
-  border-color: transparent;
-  outline: var(--focus-outline);
-  outline-offset: var(--focus-outline-inset);
-  outline-color: var(--toolbar-field-focus-border-color);
-  box-shadow: 0 1px 3px rgba(0, 0, 0, 0.23);
-}
+  &:focus {
+    background-color: var(--toolbar-field-focus-background-color);
+    color: var(--toolbar-field-focus-color);
+    border-color: transparent;
+    outline: var(--focus-outline);
+    outline-offset: var(--focus-outline-inset);
+    outline-color: var(--toolbar-field-focus-border-color);
+    box-shadow: 0 1px 3px rgba(0, 0, 0, 0.23);
+  }
 
-html|input.findbar-textbox:-moz-lwtheme::selection {
-  background-color: var(--lwt-toolbar-field-highlight, Highlight);
-  color: var(--lwt-toolbar-field-highlight-text, HighlightText);
-}
+  :root[lwtheme] &::selection {
+    background-color: var(--lwt-toolbar-field-highlight, Highlight);
+    color: var(--lwt-toolbar-field-highlight-text, HighlightText);
+  }
 
-html|input.findbar-textbox:not(:focus):-moz-lwtheme::selection {
-  background-color: var(--lwt-toolbar-field-highlight, text-select-background-disabled);
-}
+  :root[lwtheme] &:not(:focus)::selection {
+    background-color: var(--lwt-toolbar-field-highlight, text-select-background-disabled);
+  }
 
-html|input.findbar-textbox[status="notfound"] {
-  background-color: rgba(226,40,80,.3);
-  color: inherit;
-}
+  &[status="notfound"] {
+    background-color: rgba(226,40,80,.3);
+    color: inherit;
+  }
 
-html|input.findbar-textbox[flash="true"] {
-  background-color: rgba(255,233,0,.3);
-  color: inherit;
+  &[flash="true"] {
+    background-color: rgba(255,233,0,.3);
+    color: inherit;
+  }
 }
 
 /* Previous/next buttons */
@@ -119,16 +117,18 @@ html|input.findbar-textbox[flash="true"] {
     outline: var(--focus-outline);
     outline-offset: var(--focus-outline-inset);
   }
-}
 
-.findbar-find-previous:not([disabled]):hover,
-.findbar-find-next:not([disabled]):hover {
-  background-color: var(--toolbarbutton-hover-background, rgba(190,190,190,.2));
-}
+  &:not([disabled]):hover {
+    background-color: var(--toolbarbutton-hover-background, rgba(190,190,190,.2));
+  }
+
+  &:not([disabled]):hover:active {
+    background-color: var(--toolbarbutton-active-background, rgba(190,190,190,.4));
+  }
 
-.findbar-find-previous:not([disabled]):hover:active,
-.findbar-find-next:not([disabled]):hover:active {
-  background-color: var(--toolbarbutton-active-background, rgba(190,190,190,.4));
+  &[disabled="true"] > .toolbarbutton-icon {
+    opacity: var(--toolbarbutton-disabled-opacity);
+  }
 }
 
 .findbar-find-previous {
@@ -141,11 +141,6 @@ html|input.findbar-textbox[flash="true"] {
   margin-inline: 0 8px;
 }
 
-.findbar-find-previous[disabled="true"] > .toolbarbutton-icon,
-.findbar-find-next[disabled="true"] > .toolbarbutton-icon {
-  opacity: var(--toolbarbutton-disabled-opacity);
-}
-
 /* Checkboxes & Labels */
 
 .findbar-container > checkbox,
@@ -175,17 +170,13 @@ html|input.findbar-textbox[flash="true"] {
   width: 24px;
   fill: var(--toolbarbutton-icon-fill);
   transition: opacity 150ms ease-in-out;
-}
-
-findbar[hidden] > .findbar-closebutton {
-  opacity: 0;
-}
 
-.close-icon.findbar-closebutton:hover {
-  background-color: var(--toolbarbutton-hover-background, rgba(190,190,190,.2));
-  outline: none;
-}
+  &:hover {
+    background-color: var(--toolbarbutton-hover-background, rgba(190,190,190,.2));
+    outline: none;
+  }
 
-.close-icon.findbar-closebutton:hover:active {
-  background-color: var(--toolbarbutton-active-background, rgba(190,190,190,.4));
+  &:hover:active {
+    background-color: var(--toolbarbutton-active-background, rgba(190,190,190,.4));
+  }
 }
diff --git a/toolkit/themes/shared/global-shared.css b/toolkit/themes/shared/global-shared.css
index 320245afbefd3..22fd19bb83615 100644
--- a/toolkit/themes/shared/global-shared.css
+++ b/toolkit/themes/shared/global-shared.css
@@ -48,7 +48,7 @@
   --toolbar-bgcolor: var(--toolbar-non-lwt-bgcolor);
   --toolbar-color: var(--toolbar-non-lwt-textcolor);
 
-  &:-moz-lwtheme {
+  &[lwtheme] {
     --toolbar-bgcolor: rgba(255,255,255,.4);
     --toolbar-color: var(--lwt-text-color, inherit);
   }
@@ -72,7 +72,7 @@
     --panel-separator-color: currentColor;
     --toolbar-field-focus-border-color: var(--focus-outline-color);
 
-    &:not(:-moz-lwtheme) {
+    &:not([lwtheme]) {
       --panel-disabled-color: GrayText;
     }
   }
@@ -96,7 +96,7 @@
 
 /* Lightweight theme roots */
 
-:root:-moz-lwtheme {
+:root[lwtheme] {
   toolbar,
   &[lwt-popup="light"] panel {
     color-scheme: light;
diff --git a/toolkit/themes/shared/notification.css b/toolkit/themes/shared/notification.css
index b94918cce8240..8ddab7fff2003 100644
--- a/toolkit/themes/shared/notification.css
+++ b/toolkit/themes/shared/notification.css
@@ -41,15 +41,15 @@ notification {
 }
 
 @media (prefers-color-scheme: dark) {
-  notification[type="info"]:-moz-lwtheme {
-    --notification-background: #38383d;
-    --notification-text: rgb(249, 249, 250);
-  }
-
   notification[type="info"] {
     --notification-button-background: rgba(249,249,250,.1);
     --notification-button-background-hover: rgba(249,249,250,.2);
     --notification-button-background-active: rgba(249,249,250,.3);
+
+    *|*:root[lwtheme] & {
+      --notification-background: #38383d;
+      --notification-text: rgb(249, 249, 250);
+    }
   }
 }
 
diff --git a/toolkit/themes/windows/global/menu.css b/toolkit/themes/windows/global/menu.css
index 4868f41ed12f8..4889de0003c3b 100644
--- a/toolkit/themes/windows/global/menu.css
+++ b/toolkit/themes/windows/global/menu.css
@@ -51,20 +51,22 @@ menubar > menu[_moz-menuactive="true"]:not([disabled="true"]) {
   color: -moz-menubarhovertext;
 }
 
-menubar > menu:-moz-lwtheme {
-  appearance: none;
-  border-color: transparent;
+*|*:root[lwtheme] {
+  menubar > menu {
+    appearance: none;
+    border-color: transparent;
+
+    &:not([disabled="true"]) {
+      color: inherit;
+    }
+    &[_moz-menuactive="true"]:not([disabled="true"]) {
+      background-color: SelectedItem;
+      color: SelectedItemText;
+      text-shadow: none;
+    }
+  }
 }
 
-menubar > menu:-moz-lwtheme:not([disabled="true"]) {
-  color: inherit;
-}
-
-menubar > menu:-moz-lwtheme[_moz-menuactive="true"]:not([disabled="true"]) {
-  background-color: SelectedItem;
-  color: SelectedItemText;
-  text-shadow: none;
-}
 
 /* ..... internal content .... */
 
diff --git a/toolkit/themes/windows/global/toolbar.css b/toolkit/themes/windows/global/toolbar.css
index 45b79eafddf12..2b336dd0c9bca 100644
--- a/toolkit/themes/windows/global/toolbar.css
+++ b/toolkit/themes/windows/global/toolbar.css
@@ -24,9 +24,11 @@ toolbar:first-child {
   min-width: 1px;
 }
 
-toolbox:-moz-lwtheme,
-toolbar:-moz-lwtheme {
-  appearance: none;
+*|*:root[lwtheme] {
+  toolbox,
+  toolbar {
+    appearance: none;
+  }
 }
 
 toolbarseparator {
diff --git a/xpcom/ds/StaticAtoms.py b/xpcom/ds/StaticAtoms.py
index 2a0a25431ed13..9bd91f483c517 100644
--- a/xpcom/ds/StaticAtoms.py
+++ b/xpcom/ds/StaticAtoms.py
@@ -634,7 +634,6 @@
     Atom("lowest", "lowest"),
     Atom("lowsrc", "lowsrc"),
     Atom("ltr", "ltr"),
-    Atom("lwtheme", "lwtheme"),
     Atom("main", "main"),
     Atom("map", "map"),
     Atom("manifest", "manifest"),