I am a major Firefox addict. I think I was hooked at first by the tabs. I usually have at least a half dozen open at a time. What turned me into an addict though are the extensions. Too many to list really but here are the ones I use in relation to web development:
Web Developer (adds a menu & toolbar with various web developer tools)
IE Tab (enables you to use the embedded IE engine with Mozilla/Firefox)
Firebug (another web developer tool)
Colorzilla (Advanced eyedropper, color picker, screen zoom and other colorful goodies)
Foxytunes (for controlling your music player (iTunes, vlc, WMP etc) through a built in browser interface, since I always listen to music when I'm working
