ColorZilla — Advanced Eyedropper, Color Picker, Palette Viewer and other colorful goodies for your Firefox

FireFTP — FireFTP is a free, secure, cross-platform FTP client for Firefox which provides easy and intuitive access to FTP servers.

FireBug — Hands down best tool to debug CSS, HTML, and JavaScript within your browser.

Web Development Toolbar — The Web Developer extension adds a menu and a toolbar to the browser with a treasure trove of web development tools.

YSlow — YSlow analyzes your web pages and tells you why they're slow.