Backed out changeset ab00cf793f29 (bug 1800417) for causing failures at browser_unified_extensions_overflowable_toolbar.js.

browser/components/extensions/parent/ext-browserAction.js

@@ -219,7 +219,11 @@ this.browserAction = class extends ExtensionAPIPersistent {
         // Ensure the extension context menuitems are available by setting this
         // on all button children and the item.
         button.setAttribute("data-extensionid", extension.id);
-        button.classList.add("unified-extensions-item-action-button");
+        button.classList.add(
+          "toolbarbutton-1",
+          "unified-extensions-item-action-button",
+          "subviewbutton"
+        );
 
         let contents = document.createXULElement("vbox");
         contents.classList.add("unified-extensions-item-contents");