Thanks to Greg Beavers trick on debugging using Zend Server from the CLI I have come up with an equally cool (IMO) trick for Firefox.
Simply drag this link to your Personal Toolbar: Debug in ZDE
Then, edit its link (Right click > Properties) and change the debug_host to your own IP (this link has it set for localhost). Your IP must be in the zend_debugger.allow_hosts php.ini setting for this to work.
Now, you can click the link for any page you want to debug, just like in IE. This doesn’t work for posts or anything fancy (I don’t think at least, might after you post to the page) but its a start. Anybody want to write an XUL toolbar? :D