Am I supposed to get this from somewhere else?
Yes, you need to get FirePHP into your project and autoloading.
If you're using composer (and I recommend that you do), just add:
"firephp/firephp-core" : "dev-master"
(or similar) in your composer.json and update. If you're not using composer, you should grab the firephp libs, and let your autoloader know about them.