--- a/mail/base/content/mailWidgets.js
+++ b/mail/base/content/mailWidgets.js
@@ -570,17 +570,17 @@ class MozThreadPaneTreeColpicker extends
 }
 customElements.define("thread-pane-treecolpicker", MozThreadPaneTreeColpicker, { extends: "treecolpicker" });
 
 // The menulist CE is defined lazily. Create one now to get menulist defined,
 // allowing us to inherit from it.
 if (!customElements.get("menulist")) {
   delete document.createXULElement("menulist");
 }
-customElements.whenDefined("menulist").then(() => {
+{
   /**
    * MozMenulistEditable is a menulist widget that can be made editable by setting editable="true".
    * With an additional type="description" the list also contains an additional label that can hold
    * for instance, a description of a menu item.
    * It is typically used e.g. for the "Custom From Address..." feature to let the user chose and
    * edit the address to send from.
    * @extends {MozMenuList}
    */
@@ -750,17 +750,17 @@ customElements.whenDefined("menulist").t
   }
 
   const MenuBaseControl = MozElements.BaseControlMixin(MozElements.MozElementMixin(XULMenuElement));
   MenuBaseControl.implementCustomInterface(
     MozMenulistEditable, [Ci.nsIDOMXULMenuListElement, Ci.nsIDOMXULSelectControlElement]
   );
 
   customElements.define("menulist-editable", MozMenulistEditable, { extends: "menulist" });
-});
+}
 /**
  * The MozMailMultiEmailheaderfield widget shows multiple emails. It collapses long rows and allows
  * toggling the full view open. This widget is typically used in the message header pane to show
  * addresses for To, Cc, Bcc, and any other addressing type header that can contain more than one
  * mailbox.
  *
  * extends {MozXULElement}
  */

--- a/mail/components/addrbook/content/menulist-addrbooks.js
+++ b/mail/components/addrbook/content/menulist-addrbooks.js
@@ -2,18 +2,17 @@
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at https://mozilla.org/MPL/2.0/. */
 
  // The menulist CE is defined lazily. Create one now to get menulist defined,
 // allowing us to inherit from it.
 if (!customElements.get("menulist")) {
   delete document.createXULElement("menulist");
 }
-
-customElements.whenDefined("menulist").then(() => {
+{
   const { MailServices } = ChromeUtils.import("resource:///modules/MailServices.jsm");
   const { fixIterator } = ChromeUtils.import("resource:///modules/iteratorUtils.jsm");
   /**
    * MozMenulistAddrbooks is a menulist widget that is automatically
    * populated with the complete address book list.
    * @extends {MozMenuList}
    */
   class MozMenulistAddrbooks extends customElements.get("menulist") {
@@ -280,9 +279,9 @@ customElements.whenDefined("menulist").t
 
       // Sort anything else by the dir type.
       return a.dirType - b.dirType;
     }
   }
 
   customElements.define("menulist-addrbooks",
     MozMenulistAddrbooks, { extends: "menulist" });
-});
+}

--- a/mail/components/extensions/parent/ext-legacy.js
+++ b/mail/components/extensions/parent/ext-legacy.js
@@ -164,17 +164,17 @@ this.legacy = class extends ExtensionAPI
     // Overlays.load must only be called once per window per extension.
     // We use this WeakSet to remember all windows we've already seen.
     let seenWindows = new WeakSet();
 
     // Listen for new windows to overlay.
     let documentObserver = {
       observe(doc) {
         let win = doc.defaultView;
-        if (ExtensionCommon.instanceOf(doc, "XULDocument") &&
+        if (ExtensionCommon.instanceOf(doc, "HTMLDocument") &&
             !seenWindows.has(win)) {
           seenWindows.add(win);
           Overlays.load(chromeManifest, win);
         }
       },
     };
     Services.obs.addObserver(documentObserver, "chrome-document-interactive");