Hey, why don't we share favorite Firefox extensions here?
Measure It- A draw out ruler for getting pixel height and width of elements on your screen.
Bandwidth Tester- obvious
Add Bookmark Here- makes bookmark management easier.
FoxyTunes- control your music from your browser.
Autocopy- highlighting text autocopies it, like in IRC.
Colorzilla- eyedropper for your browser to pick up colors.
DictionarySearch- highlight text and adds a dictinarysearch to your right click dropdown .
IE View- for when sites are built wrong.
Forecast Fox- tells me the weather in that bottom bar.
Download Manager Tweak- This makes the downloading manager open in a tab instead of a window.
BBCode Xtra- for making things easier in places like this.
BugMeNot- for reading online newspapers without filling out forms.
ImageZoom- Right click on an image and mousewheel zoom it in and out.
Tab Mix Plus- Best tab manager I've seen.