jQuery doesn't work fully on a Firefox addon

I've included jQuery by doing this:

<?xml version="1.0"?>
<!DOCTYPE window SYSTEM "chrome://orkutmanager/locale/browser.dtd">
    <overlay id="omcore" 
        开发者_Go百科 xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">

    <script type="application/x-javascript" src="../../jquery-1.4.2.js"/>
    <script type="text/javascript">jQuery.noConflict();</script>

    <script type="application/x-javascript" src="../../OM.js"/>

</overlay>

Here is my chrome.manifest

content orkutmanager    chrome/content/
overlay chrome://browser/content/browser.xul chrome://orkutmanager/content/XUL/Browser/Core.xul

skin orkutmanager classic skin/

locale orkutmanager en-US locale/en-US/

And then I tried to use

gBrowser.addEventListener('DOMContentLoaded',
    function (e)
    {
        var contentWindow = e.originalTarget.defaultView;
        var contentDocument = contentWindow.document;

        var $ = jQuery;

        var x = $("a", contentDocument).attr("onclick"); //
        // Error: uncaught exception: unknown (can't convert to string)

    }, false);

While

$("a", contentDocument).get(0).getAttribute("onclick");

Works normal.

Do I have to include jQuery in a different way? How can I make it work as usual? Is there another jQuery method to get the value of an attribute?

On a normal case it works, example here.

---

You are accessing the onclick property using the attr function.

The docs say nothing about it, but could it be that attr() forces its return value into a string? It looks like it.

What if you access onclick directly? What are you doing with it?