While working on a Songbird extension, I found myself needing to debug some AJAX code. Songbird is built on the Mozilla application framework, and I haven’t yet figured out how to do this kind of debugging directly in the application, so I just ripped out the relevant code and put it into a tst.html file so I could debug it with Firebug in Firefox.
Put this in your page, and it’ll pop up a window asking you whether to allow XSS for the page. It has a checkbox to remember your choice, so it only prompts you the first time. Perfect!