Fixing failure on firefox documentXUL when you tried to override document.head

Author: guilhermef

processing.js

@@ -28,7 +28,9 @@
   var isDOMPresent = ("document" in this) && !("fake" in this.document);
 
   // document.head polyfill for the benefit of Firefox 3.6
-  document.head = document.head || document.getElementsByTagName('head')[0];
+  if (!document.head) {
+    document.getElementsByTagName('head')[0];
+  }
 
   // Typed Arrays: fallback to WebGL arrays or Native JS arrays if unavailable
   function setupTypedArray(name, fallback) {