How to debug Firefox extension

I've been into Firefox extension development recently, and ran into some issues:

So, in browser.xul i defined these lines:

<overlay id="sample" xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
    <script src="jquery.js" />
    <script src="global.js" />
</overlay>

So, in global.js i have access to all jQuery stuff, and trying to load a simple script there:

var inner = null;
var o = function () {
    var prefManager = Components.classes["@mozilla.org/preferences-service;1"].getService(Components.interfaces.nsIPrefBranch);
    return {
        init : function () {
            alert('here loading inner..');
            $.get('http://www.example.com/script.js', function(d) {
                alert('loaded inner script!');
                inner = d;
                gBrowser.addEventListener("load", function () {
                    alert('onload');
                }, false);
            }).error(function(e) { alert('error loading inner..'); setTimeout(o.init,1000); });
            $(this).ajaxError(function() { alert('ajaxError'); });
        }
    }
}
window.addEventListener("load", o.init, false);

But nor i receive a "loaded inner script", nor a "error loading inner" alert.. And i don't see the error console to log any errors from the extension... I assume the $.get is silently failing due to some restrictions maybe, but is there a proper way to debug the errors normally? The error console is silent for the extension, it only shows errors from the web pages

If you look at the article Setting up an extension development environment it suggests setting up some preferences, including javascript.options.showInConsole = true, which logs errors in chrome files to the Error Console.