--- a/browser/components/customizableui/CustomizableUI.jsm
+++ b/browser/components/customizableui/CustomizableUI.jsm
@@ -2764,16 +2764,20 @@ var CustomizableUIInternal = {
    * @return {Boolean} whether the widget is removable
    */
   isWidgetRemovable(aWidget) {
     let widgetId;
     let widgetNode;
     if (typeof aWidget == "string") {
       widgetId = aWidget;
     } else {
+      // Skipped items could just not have ids.
+      if (!aWidget.id && aWidget.getAttribute("skipintoolbarset") == "true") {
+        return false;
+      }
       if (!aWidget.id &&
           !["toolbarspring", "toolbarspacer", "toolbarseparator"].includes(aWidget.nodeName)) {
         throw new Error("No nodes without ids that aren't special widgets should ever come into contact with CUI");
       }
       // Use "spring" / "spacer" / "separator" for special widgets without ids
       widgetId = aWidget.id || aWidget.nodeName.substring(7 /* "toolbar".length */);
       widgetNode = aWidget;
     }