Greasemonkey is a firefox extension that allows you to customize the way a webpage displays using small bits of JavaScript

You can download the extension here ;

https://addons.mozilla.org/nl/firefox/addon/748

For scripts goto

http://userscripts.org/

Popularity: 2%

One of the first things I install after I install firefox. This is a great firefox extension !

Install Xmarks on each computer you use, and it seamlessly integrates with your web browser and keeps your bookmarks safely backed up and in sync.

http://www.xmarks.com

Popularity: 1%