How to debug Firefox extension

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.

In general it can be problematic using JQuery in a XUL page since it assumes that the document is an HTML DOM rather than an XML DOM and that the window is a HTML window rather than a XUL window. If I were you I'd use the subscript loader for this. To debug you can use Venkman although it is a bit flakey and I often resort to just dump() statements to the console instead.

Update: see my comment below about the Browser Toolbox.