Mine would have to be mouse gestures. Either All-in-one gestures or FireGestures

You just made me realize that there's no extention I couldn't live without!! Maybe I don't have the right ones myself...

I don't use any

just downloaded firefox this week for the first time and don't have any.

pleasant. wow I'm surprised you use the gestures plugin. I hated the feel of it. Cool though. ummmm for me it would be Read It Later

mine would have to be Stumble Upon

sxiper , del.icio.us

foxmarks.

I can't live without Try Again firefox addon

Try Again. I'll have to check that one out.