I've personally got the extensions for TinyURL, BugMeNot, Sage, & DictionarySearch running on my Firefox installation. I will most certainly now be adding a few more.

"One of its strengths is the active development community that works to expand and improve the browser's functionality through the development of Firefox extensions -- small applications that add functionality to Firefox. Here are a few of my favorite extensions, all tested with Firefox 1.0.1 on Windows XP and Mandrakelinux."