FirePHP is really an extension to an extension. You have to have the Firebug extension installed for FirePHP to work correctly. You also have to download and install the FirePHP class to your website.
You can get the extension and learn all about it here.
This extension allows me to write to the Firebug console window using PHP. It allows me to view my php variables and objects in the Firebug and FirePHP windows.
I can also set a variable to disable FirePHP for production. Also Very Cool!
FirePHP is a static class so once I’ve got everything installed and the classes included correctly, I can write something like this anywhere in my code and get a sweet message in my Firebug console.
FB::log('My Sweet Message Here');
There are several options I can use with FirePHP. I especially like that I can send log, info, warn, or error messages and they will be marked differently in the console.
Now, maybe the designers and clients I work with will quit fussing at me for leaving my debug code on the screen.