Greasemonkey is a Firefox extension that allows you to customize the way webpages look and function

Fine-tune Firefox with this extension, which lets you add in more scripts for tinkering and tweaking. Greasemonkey improves just about anything it touches, whether it's adding must-have features to Gmail, streamlining your social life in Facebook, or speeding up your blog posts The best part? Thousands of Greasemonkey scripts are freely available for download, and installing them is literally as simple as clicking a single link...Greasemonkey Homepage