What I love about Mozilla Firefox is all the great extensions for it. 

This list supercedes the original list at Favorite Firefox Extensions

Items in red are new to the list.

Adbloc Filterset.G Updater
Adblock Plus
AI Roboform Toolbar
ColorZilla
DOM Inspector
Dictionary Search
Download Manager Tweak
FireBug
IE Tab
Image Zoom
JavaScript Debugger
Map+
MeasureIt
Nuke Anything Enhanced
QuickRestart
Split Pannel
Tab Mix Plus
Talkback
View Cookies
Web Developer
wmlbrowser
Yahoo! Toolbar

IE View Replaced by IE Tab
Nuke Anything Replace by Nuke Anything Enhanced
Clone Window Replaced by Firefox 2.0
Adblock Replaced by Adblock Plus
Tabbrowser Preferences
BlogJetThis! Use Windows Live Writer
Greasemonkey Too many issues