diff --git a/dom/locales/en-US/chrome/dom/dom.properties b/dom/locales/en-US/chrome/dom/dom.properties
index 2117887e4c18e..e7df4a40699e2 100644
--- a/dom/locales/en-US/chrome/dom/dom.properties
+++ b/dom/locales/en-US/chrome/dom/dom.properties
@@ -212,6 +212,8 @@ ServiceWorkerGraceTimeoutTermination=Terminating ServiceWorker for scope ‘%1$S
 # LOCALIZATION NOTE (ServiceWorkerNoFetchHandler): Do not translate "Fetch".
 ServiceWorkerNoFetchHandler=Fetch event handlers must be added during the worker script’s initial evaluation.
 ExecCommandCutCopyDeniedNotInputDriven=document.execCommand(‘cut’/‘copy’) was denied because it was not called from inside a short running user-generated event handler.
+ManifestIdIsInvalid=The id member did not resolve to a valid URL.
+ManifestIdNotSameOrigin=The id member must have the same origin as the start_url member.
 ManifestShouldBeObject=Manifest should be an object.
 ManifestScopeURLInvalid=The scope URL is invalid.
 ManifestScopeNotSameOrigin=The scope URL must be same origin as document.
diff --git a/dom/manifest/ManifestProcessor.jsm b/dom/manifest/ManifestProcessor.jsm
index b551f5c25a700..6830b80b66ac6 100644
--- a/dom/manifest/ManifestProcessor.jsm
+++ b/dom/manifest/ManifestProcessor.jsm
@@ -124,6 +124,7 @@ var ManifestProcessor = {
       background_color: processBackgroundColorMember(),
     };
     processedManifest.scope = processScopeMember();
+    processedManifest.id = processIdMember();
     if (checkConformance) {
       processedManifest.moz_validation = errors;
       processedManifest.moz_manifest_url = manifestURL.href;
@@ -258,10 +259,10 @@ var ManifestProcessor = {
         expectedType: "string",
         trim: false,
       };
-      let result = new URL(docURL).href;
+      const defaultStartURL = new URL(docURL).href;
       const value = extractor.extractValue(spec);
       if (value === undefined || value === "") {
-        return result;
+        return defaultStartURL;
       }
       let potentialResult;
       try {
@@ -269,17 +270,16 @@ var ManifestProcessor = {
       } catch (e) {
         const warn = domBundle.GetStringFromName("ManifestStartURLInvalid");
         errors.push({ warn });
-        return result;
+        return defaultStartURL;
       }
       if (potentialResult.origin !== docURL.origin) {
         const warn = domBundle.GetStringFromName(
           "ManifestStartURLShouldBeSameOrigin"
         );
         errors.push({ warn });
-      } else {
-        result = potentialResult.href;
+        return defaultStartURL;
       }
-      return result;
+      return potentialResult.href;
     }
 
     function processThemeColorMember() {
@@ -314,6 +314,42 @@ var ManifestProcessor = {
       };
       return extractor.extractLanguageValue(spec);
     }
+
+    function processIdMember() {
+      // the start_url serves as the fallback, in case the id is not specified
+      // or in error. A start_url is assured.
+      const startURL = new URL(processedManifest.start_url);
+
+      const spec = {
+        objectName: "manifest",
+        object: rawManifest,
+        property: "id",
+        expectedType: "string",
+        trim: false,
+      };
+      const extractedValue = extractor.extractValue(spec);
+
+      if (typeof extractedValue !== "string" || extractedValue === "") {
+        return startURL.href;
+      }
+
+      let appId;
+      try {
+        appId = new URL(extractedValue, startURL.origin);
+      } catch {
+        const warn = domBundle.GetStringFromName("ManifestIdIsInvalid");
+        errors.push({ warn });
+        return startURL.href;
+      }
+
+      if (appId.origin !== startURL.origin) {
+        const warn = domBundle.GetStringFromName("ManifestIdNotSameOrigin");
+        errors.push({ warn });
+        return startURL.href;
+      }
+
+      return appId.href;
+    }
   },
 };
 var EXPORTED_SYMBOLS = ["ManifestProcessor"];
diff --git a/dom/manifest/test/mochitest.ini b/dom/manifest/test/mochitest.ini
index c496caed42b80..73d7a6578707f 100644
--- a/dom/manifest/test/mochitest.ini
+++ b/dom/manifest/test/mochitest.ini
@@ -13,6 +13,7 @@ support-files =
 [test_ManifestProcessor_dir.html]
 [test_ManifestProcessor_display.html]
 [test_ManifestProcessor_icons.html]
+[test_ManifestProcessor_id.html]
 [test_ManifestProcessor_JSON.html]
 [test_ManifestProcessor_lang.html]
 [test_ManifestProcessor_name_and_short_name.html]
diff --git a/dom/manifest/test/test_ManifestProcessor_id.html b/dom/manifest/test/test_ManifestProcessor_id.html
new file mode 100644
index 0000000000000..336d1d3a773bd
--- /dev/null
+++ b/dom/manifest/test/test_ManifestProcessor_id.html
@@ -0,0 +1,123 @@
+<!DOCTYPE HTML>
+<html>
+<!--
+https://bugzilla.mozilla.org/show_bug.cgi?id=1731940
+-->
+<head>
+  <meta charset="utf-8">
+  <title>Test for Bug 1731940 - implement id member</title>
+  <script src="/tests/SimpleTest/SimpleTest.js"></script>
+  <link rel="stylesheet" type="text/css" href="/tests/SimpleTest/test.css"/>
+  <script src="common.js"></script>
+  <script>
+  /**
+   * Manifest id member
+   * https://w3c.github.io/manifest/#id-member
+   **/
+  for (const type of typeTests) {
+    data.jsonText = JSON.stringify({
+      id: type,
+    });
+    const result = processor.process(data);
+    is(
+      result.id.toString(),
+      result.start_url.toString(),
+      `Expect non-string id to fall back to start_url: ${typeof type}.`
+    );
+  }
+
+  // Invalid URLs
+  const invalidURLs = [
+    "https://foo:65536",
+    "https://foo\u0000/",
+    "//invalid:65555",
+    "file:///passwords",
+    "about:blank",
+    "data:text/html,<html><script>alert('lol')<\/script></html>",
+  ];
+
+  for (const url of invalidURLs) {
+    data.jsonText = JSON.stringify({
+      id: url,
+    });
+    const result = processor.process(data);
+    is(
+      result.id.toString(),
+      result.start_url.toString(),
+      "Expect invalid id URL to fall back to start_url."
+    );
+  }
+
+  // Not same origin
+  data.jsonText = JSON.stringify({
+    id: "http://not-same-origin",
+  });
+  var result = processor.process(data);
+  is(
+    result.id.toString(),
+    result.start_url,
+    "Expect different origin id to fall back to start_url."
+  );
+
+  // Empty string test
+  data.jsonText = JSON.stringify({
+    id: "",
+  });
+  result = processor.process(data);
+  is(
+    result.id.toString(),
+    result.start_url.toString(),
+    `Expect empty string for id to use start_url.`
+  );
+
+  // Resolve URLs relative to the start_url's origin
+  const URLs = [
+    "path",
+    "/path",
+    "../../path",
+    "./path",
+    `${whiteSpace}path${whiteSpace}`,
+    `${whiteSpace}/path`,
+    `${whiteSpace}../../path`,
+    `${whiteSpace}./path`,
+  ];
+
+  for (const url of URLs) {
+    data.jsonText = JSON.stringify({
+      id: url,
+      start_url: "/path/some.html",
+    });
+    result = processor.process(data);
+    const baseOrigin = new URL(result.start_url.toString()).origin;
+    const expectedUrl = new URL(url, baseOrigin).toString();
+    is(
+      result.id.toString(),
+      expectedUrl,
+      "Expected id to be resolved relative to start_url's origin."
+    );
+  }
+
+  // Handles unicode encoded URLs
+  const specialCases = [
+    ["😀", "%F0%9F%98%80"],
+    [
+      "this/is/ok?query_is_ok=😀#keep_hash",
+      "this/is/ok?query_is_ok=%F0%9F%98%80#keep_hash",
+    ],
+  ];
+  for (const [id, expected] of specialCases) {
+    data.jsonText = JSON.stringify({
+      id,
+      start_url: "/my-app/",
+    });
+    result = processor.process(data);
+    const baseOrigin = new URL(result.start_url.toString()).origin;
+    const expectedUrl = new URL(expected, baseOrigin).toString();
+    is(
+      result.id.toString(),
+      expectedUrl,
+      `Expect id to be encoded/decoded per URL spec.`
+    );
+  }
+  </script>
+</head>
diff --git a/dom/manifest/test/test_ManifestProcessor_start_url.html b/dom/manifest/test/test_ManifestProcessor_start_url.html
index af9662d57d6a8..1d172df555497 100644
--- a/dom/manifest/test/test_ManifestProcessor_start_url.html
+++ b/dom/manifest/test/test_ManifestProcessor_start_url.html
@@ -40,9 +40,11 @@
 result = processor.process(data);
 is(result.start_url.toString(), docURL.toString(), expected);
 
-
-// Resolve URLs relative to manfiest
-var URLs = ["path", "/path", "../../path",
+// Resolve URLs relative to manifest
+var URLs = [
+  "path",
+  "/path",
+  "../../path",
   `${whiteSpace}path${whiteSpace}`,
   `${whiteSpace}/path`,
   `${whiteSpace}../../path`,
@@ -58,5 +60,24 @@
   is(result.start_url.toString(), absURL, expected);
 });
 
-  </script>
-</head>
+// It retains the fragment
+var startURL = "./path?query=123#fragment";
+data.jsonText = JSON.stringify({
+  start_url: startURL,
+});
+var absURL = new URL(startURL, manifestURL).href;
+result = processor.process(data);
+is(result.start_url.toString(), absURL, "Retains fragment");
+
+// It retains the fragment on the document's location too.
+window.location = "#here";
+data.jsonText = JSON.stringify({});
+result = processor.process(data);
+is(
+  window.location.href,
+  result.start_url.toString(),
+  `Retains the fragment of document's location`
+);
+</script>
+  </head>
+</html>
diff --git a/dom/manifest/test/test_ManifestProcessor_warnings.html b/dom/manifest/test/test_ManifestProcessor_warnings.html
index 04e0c2eddbd27..f2092c1dcf2f6 100644
--- a/dom/manifest/test/test_ManifestProcessor_warnings.html
+++ b/dom/manifest/test/test_ManifestProcessor_warnings.html
@@ -112,6 +112,25 @@
     },
     warn: "icons item at index 1 includes repeated purpose(s): any maskable.",
   },
+  // testing dom.properties: ManifestIdIsInvalid
+  {
+    func() {
+      return (options.jsonText = JSON.stringify({
+        id: "http://test:65536/foo",
+      }));
+    },
+    warn: "The id member did not resolve to a valid URL.",
+  },
+  // testing dom.properties ManifestIdNotSameOrigin
+  {
+    func() {
+      return (options.jsonText = JSON.stringify({
+        id: "https://other.com",
+        start_url: "/this/place"
+      }));
+    },
+    warn: "The id member must have the same origin as the start_url member.",
+  }
 ].forEach((test, index) => {
   test.func();
   const result = processor.process(options);