Bug 1805360 - Fix FxA panel separators. r=mconley

Replace menuseparator -> toolbarseparator in the FxA panel.

Differential Revision: https://phabricator.services.mozilla.com/D164537

browser/base/content/browser-sync.js

@@ -151,7 +151,7 @@ this.SyncedTabsPanelList = class SyncedTabsPanelList {
         for (let client of clients) {
           // add a menu separator for all clients other than the first.
           if (fragment.lastElementChild) {
-            let separator = document.createXULElement("menuseparator");
+            let separator = document.createXULElement("toolbarseparator");
             fragment.appendChild(separator);
           }
           // We add the client's elements to a container, and indicate which