Here are the Firefox extensions that I have installed right now:
Filters ads from web pages.
Bypasses compulsory web registrations at sites like The New York Times.
Downloads *.mp3, *.jpg, or any other filetype linked from a web page with just one click.
The greatest, simplest RSS/Atom newsreader in the world.
Adds thumbnails to Google search results (and all the other major search engines.)
Puts ad-free weather forecasts in your toolbar. Super customizable.
Browser extensions are wonderful things!