My list
Yahoo Mail Notifier (not the one from Yahoo, so no toolbar!)
Adblock Plus (adds a button to my existing toolbar)
Enhanced History Manager (also adds a button. I like it for the separate window option -- I have an aversion to sidebars.)
Flashblock
Forecastfox (connected to my old employers, Accuweather)
ImageShack right-click (I don't use it much, but it comes in handy for posting to forums)
PDF Download (gives you the option of opening a PDF or downloading it for separate viewing later -- default is to open, which can slow things down considerably)
SmoothWheel (a scrolling aid for the mouse)
...and finally, Talkback... which is the only extension I know of that loads itself with Firefox, enabling the reporting of problems back to Mozilla.