--- a/browser/base/content/browser-social.js
+++ b/browser/base/content/browser-social.js
@@ -357,17 +357,17 @@ SocialShare = {
     }
     this.defaultButton.setAttribute("checked", "true");
   },
 
   get shareButton() {
     // web-panels (bookmark/sidebar) don't include customizableui, so
     // nsContextMenu fails when accessing shareButton, breaking
     // browser_bug409481.js.
-    if (!window.CustomizableUI)
+    if (document.documentElement.getAttribute("windowtype") !== "navigator:browser")
       return null;
     let widget = CustomizableUI.getWidget("social-share-button");
     if (!widget || !widget.areaType)
       return null;
     return widget.forWindow(window).node;
   },
 
   _onclick() {