Davey Shafik has shared a method he's come up with for debugging directly from Firefox with the help of xdebug and the Komodo IDE.

In an effort to switch away from Zend Studio I have been trying out Komodo (again) [...] One of the last barriers to switching, has been to emulate the Zend Studio Toolbar for Firefox (also available for IE) which lets you do lots of great things, from Profiling to Debugging every page. I personally use it mostly for debugging single pages, and forms, as well as for end to end PHP and AJAX debugging - so this was the feature I was mostly looking to emulate.

He found his way through a few small additions to his php.ini file that allow Komodo to connect directly to the remote server and make debugging as easy as adding "?XDEBUG_SESSION_START=1" to the URL of whatever page you're on. He's also made a bookmarklet that turns it on for you as well.