Bug 1944016 - Add new strings to tab context menu for switching to/from vertical tabs r=sidebar-reviewers,fluent-reviewers,tabbrowser-reviewers,dao,bolsson,nsharpley

Differential Revision: https://phabricator.services.mozilla.com/D236035

browser/components/tabbrowser/content/tabbrowser.js

@@ -8470,6 +8470,17 @@ var TabContextMenu = {
         : "close-tabs-to-the-end"
     );
 
+    // Update context menu item for "Turn (on/off) Vertical Tabs".
+    const toggleVerticalTabsItem = document.getElementById(
+      "context_toggleVerticalTabs"
+    );
+    document.l10n.setAttributes(
+      toggleVerticalTabsItem,
+      gBrowser.tabContainer?.verticalMode
+        ? "tab-context-disable-vertical-tabs"
+        : "tab-context-enable-vertical-tabs"
+    );
+
     // Disable "Close Tabs to the Left/Right" if there are no tabs
     // preceding/following it.
     let noTabsToStart = !gBrowser._getTabsToTheStartFrom(this.contextTab)