Here are the Firefox extensions that I have installed right now:

Adblock
Filters ads from web pages.

BugMeNot
Bypasses compulsory web registrations at sites like The New York Times.

downTHEMall!
Downloads *.mp3, *.jpg, or any other filetype linked from a web page with just one click.

Sage
The greatest, simplest RSS/Atom newsreader in the world.

BetterSearch
Adds thumbnails to Google search results (and all the other major search engines.)

SomethingAwful Last Read Enhancement
For readers of the SomethingAwful forums. I wish every web forum used the enhancements included in this extension!

ForecastFox
Puts ad-free weather forecasts in your toolbar. Super customizable.

Browser extensions are wonderful things!