Search Engine Wizard
Lets you add your own searchengines to the searchbar in FF, just rightclick the text input box and add. Simple and usefull.
WorldClocks
This one is for all of you who like me are dealing with people and sites all over the world. Adds a clock in the statusbar that you can set to any timezone by major cities you want. Simple but almost ridiculously usefull.
FavLoc
Saving stuff to several different places on your harddrive all the time? Tired of constantly having to browse to the locations? This is for you, it adds a favourite location box to the save box and to the rightclick menu that you can modify however you want.
Internote
Fun little extension that lets you add posit's to webpages. Very usefull when doing reaserch and want to quickly sum up the different points each text has so you don't have to go back and re-read the entire thing.