The Web Developer extension adds various web developer tools to the browser.
Advanced Eyedropper, Color Picker, Gradient Generator and other colorful goodies...
YSlow analyzes web pages and suggests ways to improve their performance based on a set of rules for high performance web pages.
http://yslow.org is the official open source website.
FirePHP enables you to log to your Firebug Console using a simple PHP method call.
Not using Firebug 1.9+ on Firefox 8+? See http://firephp.org/
Finds HTML elements matching chosen CSS selector(s) or XPath expression
Firebug extension to view which CSS rules are actually used.
Firecookie is an extension for Firebug that makes possible to view and manage cookies in your browser
Firecookie is part of Firebug since Firebug 1.10