util.js: detect firefox 4.X correctly.

Update the browser detection code (from mootools) with a fix that
detects firefox 4.0 correctly.

include/util.js

@@ -244,7 +244,7 @@ Util.Engine = {
     //'webkit': (function() {
     //        return ((typeof navigator.taintEnabled !== "unknown") && navigator.taintEnabled) ? false : ((Util.Features.xpath) ? ((Util.Features.query) ? 525 : 420) : 419); }()),
     'gecko': (function() {
-            return (!document.getBoxObjectFor && !window.mozInnerScreenX) ? false : ((document.getElementsByClassName) ? 19 : 18); }())
+            return (!document.getBoxObjectFor && window.mozInnerScreenX == null) ? false : ((document.getElementsByClassName) ? 19 : 18); }())
 };
 
 Util.Flash = (function(){