With over a thousand add-ons available, there's something for
everyone. To get you started, here's a list of some of our
favorites. Enjoy!
Greasemonkey 
Allows you to customize the way a webpage displays using small bits
of JavaScript.
Hundreds of scripts, for a wide variety of poular sites, are
already available at http://userscripts.org.
You can write your own scripts, too. Mark Pilgrim's definitive
Greasemonkey guide, diveintogreasemonkey.org will show you
how.
Works with:
Firefox 1.5 - 2.0.0.* ALL
Details
The other available add-ons available, click here