Bug 356419. LABEL of PANEL and PANEL don't have LABEL_FOR/BY relations. Patch by Nian Liu. r=mano, aaronlev.

toolkit/content/widgets/groupbox.xml

@@ -44,6 +44,15 @@
                    xbl:inherits="default,value=label,crop,accesskey"/>
       </children>
     </content>
+
+    <implementation>
+      <constructor>
+        if (this.parentNode.id) {
+          document.getAnonymousNodes(this)[1]
+                  .setAttribute("control", this.parentNode.id);
+        }
+      </constructor>
+    </implementation>
   </binding>
 
 </bindings>