Firebug looks interesting, I'll have to look into that.
My personal favs:
Adblock Plus - blocks ads
User Agent Switcher - allows you to fake your browser ID. This does various things, but most useful for UW students: if you set your browser to appear non windows allows you to bypass UW wireless's requirement for Windows users to run minuet to get full port access.
All-in-One Gestures - a decent program for firefox gestures (basically, you can navigate between tabs by right clicking and dragging. it can make mouse only browsing considerable faster)