getting the HEAD element from overlay.js firefox plugin

I have a lightwiehgt plugin to firefox which needs to inject a script into the HTML. The code looks like this:

var head = document.getElementsByTagName("head")[0];
var newsc开发者_如何学Gorpt;       
newscrpt = document.createElement('script');
newscrpt.type = "text/javascript" ;
newscrpt.src = "http://blabla.com/...";
newscrpt = head.appendChild(newscrpt);

The problem is that document.getElementsByTagName("head")[0] returns 'undefined', and checking document.getElementsByTagName("head").length is 0.

It currently executes on the browser document.onLoad event but I also tried calling it from window.setTimeout to make sure it is not a problem with loading synchronization, but the same happens.

Any ideas from anyone? Thanks!

---

If you're using a frame or an iframe object, you should not reference the document directly but do something like:

    var doc = frame.contentWindow.document;

After that you can get the head. I'm using jQuery to add some resources to it:

    $(doc).find('head').append(
                '<link type="text/css" rel="stylesheet" href="MyStylesheet.css" />');

Hope it helps.