Stefan Esser has released a patch, pulled originally from the Suhosin project's functionality, that allows you to use a bit of stealth for run the Zend Debugger and XDebug at the same time.

Those asking usually can understand why running two debuggers at the same time will lead to problems, but they don't understand why problems should arise when only one is used at a time. They want to be able to load Zend Debugger and Xdebug with the same php.ini without the need to restart the whole server just to change the debugger.

You can grab the patch from his site and install it like normal. With the patch, your PHP install will handle the rest behind the scenes.

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.