NoScript is awesome. Really simple to use for an idiot (me!). A few times its save me from malicious wersites.
In addition to the obvious I use
FoxyProxy for cheating on online games,
Download Helper for stealing videos,
TVU webplayer for watching certain online feeds I'm not allowed to talk about on CP,
Download Statusbar for the status of my downloads,
CookieCuller for culling cookies,
Chatzilla for IRC chatting.
Yoono sounds cool, I'll get that one and try it out later.