To compliment his vidcast on the Zend Developer Zone, Davey Shafik has a few more helpful notes posted to his blog on debugging with Firebug.

Cal Evans at the Zend DevZone just posted my screencast on AJAX and PHP End-to-End Debugging. I mentioned in the screencast that I you could find some information on my blog that is used in the video, so this is that post.

He points out another quick method for debugging an ajax application (as mentioned by Greg Beaver) - adding the GET request values directly to the query string inside the Javascript of your application.