This is a common webish pattern to ensure that `undefined` really means undefined: in ES3, `undefined` is mutable… which can lead to stupid things.

```javascript
(function(window, document, undefined) {
  // …
})(window, document);
```

http://stackoverflow.com/questions/5020479/what-advantages-does-using-functionwindow-document-undefined-windo

In Firefox OS we don’t care because `undefined` is non-mutable, but I like the idea of applying common webish patterns wherever it doesn’t harm us. If this IIFE didn’t have any `undefined` argument I wouldn’t have asked to insert one either — my remark was only to minimize the noise.