diff --git a/modules/utils.js b/modules/utils.js
index c65859ec..9f73d5cf 100644
--- a/modules/utils.js
+++ b/modules/utils.js
@@ -493,7 +493,9 @@ var TreeStyleTabUtils = {
 		while (targets.hasMoreElements())
 		{
 			let target = targets.getNext()
-							.QueryInterface(Ci.nsIDOMWindowInternal);
+							.QueryInterface(Ci.nsIDOMWindow);
+			if ('nsIDOMWindowInternal' in Ci) // for Firefox 7 or olders
+				target = target.QueryInterface(Ci.nsIDOMWindowInternal);
 			windows.push(target);
 		}