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/
CodeBurner is a Firefox add-on that integrates with Firebug, to extend it with reference material for HTML and CSS.
Finds HTML elements matching chosen CSS selector(s) or XPath expression
Pixel Perfect is a Firefox/Firebug extension that allows web developers and designers to easily overlay a web composition over top of the developed HTML.
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