The absolute single most useful Add On I have installed. I can never imagine being without my beloved Greasemonkey.

Greasemonkey is a plugin for the Firefox web browser. It allows you to change how your favorite pages behave and look. There are many scripts that have already been written, and if you know javascript you can easily create your own!

My favorite site to install scripts from is:

http://userscripts.org/

Enjoy!