Update: This post is outdated, I created a Firefox Addon collection
I use only 4 firefox Addons. (I am guessing this may come down to 3 with FF3.1's Tabbing features). These four make the slickest and most productive FF for me.
Firebug - A Must for all Web Developers. One stop for all development.
Tab Mix Plus - Well there is some anticipation on the tabbing features coming in 3.1, but until then (or may be even after..), this one is a must have.
DownThemAll! - The best download manager. Very nice replacement for the not-so-friendly defaulter.
GrandCentral Click to Call - For those who use grandcentral, this is awesome. To be able to click a number on a webpage and place a call is one step closer to getting dead lazy