Remove the element from the id map when it's removed from the document. b=441994 r+sr=roc

content/xul/document/src/nsXULDocument.cpp

@@ -1740,8 +1740,11 @@ nsXULDocument::RemoveSubtreeFromDocument(nsIContent* aElement)
             return rv;
     }
 
-    // 2. Remove the element from the resource-to-element map
+    // 2. Remove the element from the resource-to-element map.
+    // Also remove it from the id map, since we added it in
+    // AddElementToDocumentPre().
     RemoveElementFromRefMap(aElement);
+    RemoveFromIdTable(aElement);
 
     // 3. If the element is a 'command updater', then remove the
     // element from the document's command dispatcher.