FirePHP allows you to take a deeper look at all the work your PHP code does to generate that page you are currently looking at in your Firefox browser.
Just install the FirePHP Firefox extension, PHP PEAR package and add a few lines of code to your PHP application to get started. You will see a "FirePHP" tab in your Firebug extension to bring your PHP backend logic to your fingertips!
You can check out screenshots of it in action, a demo of its use, and of course, download the latest version all from their site. It works either as a separate window or attached in your browser and, based on an XML capabilities file, works with your application to provide details about your scripts.