Fix initial letter avatar vertical offset in Firefox

res/css/views/avatars/_BaseAvatar.scss

@@ -16,6 +16,16 @@ limitations under the License.
 
 .mx_BaseAvatar {
     position: relative;
+    // In at least Firefox, the case of relative positioned inline elements
+    // (such as mx_BaseAvatar) with absolute positioned children (such as
+    // mx_BaseAvatar_initial) is a dark corner full of spider webs. It will give
+    // different results during full reflow of the page vs. incremental reflow
+    // of small portions. While that's surely a browser bug, we can avoid it by
+    // using `inline-block` instead of the default `inline`.
+    // https://github.com/vector-im/riot-web/issues/5594
+    // https://bugzilla.mozilla.org/show_bug.cgi?id=1535053
+    // https://bugzilla.mozilla.org/show_bug.cgi?id=255139
+    display: inline-block;
 }
 
 .mx_BaseAvatar_initial {