At the moment, I like the extensions that add greater capability of Firefox, like
- Iget (downloads any file given the URL)
- ColorZilla (Advanced Eyedropper, Page Zooming, Colorpicker, etc tool)
- Firefox View (Loads pages from IE into Firefox)
- IEView (Loads pages from Firefox into IE)
- ImageZoom (Zooms in on images)
- Show Image (Similar to the IE feature if an image doesn't load)
- CountdownClock Extension (this one will create a countdown clock in the status bar, which I usually use to countdown to FIRST events)
- PrintPreview (makes a nice print preview feature)
And of course I also use Adblock and the CD search extensions (thanks Dave Scheck)
