--- a/dom/base/UseCounters.conf
+++ b/dom/base/UseCounters.conf
@@ -35,16 +35,17 @@
 // dependencies were correct would have been rather difficult, and
 // annotating the WebIDL files does nothing for identifying CSS
 // property usage, which we would also like to track.
 
 method SVGSVGElement.getElementById
 attribute SVGSVGElement.currentScale
 property Fill
 property FillOpacity
+attribute XMLDocument.async
 
 // Push API
 method PushManager.subscribe
 method PushSubscription.unsubscribe
 
 // window.sidebar.addSearchEngine
 attribute Window.sidebar
 method External.addSearchEngine

--- a/dom/webidl/XMLDocument.webidl
+++ b/dom/webidl/XMLDocument.webidl
@@ -14,10 +14,11 @@ interface XMLDocument : Document {};
 // http://www.whatwg.org/specs/web-apps/current-work/#xmldocument
 partial interface XMLDocument {
   [Throws, NeedsCallerType]
   boolean load(DOMString url);
 };
 
 // Gecko extensions?
 partial interface XMLDocument {
+  [UseCounter]
   attribute boolean async;
 };