Greasemonkey

Greasemonkey is a firefox extension that lets you write scripts that alter the web pages you visit. With greasemonkey you can fix bugs, automatically retrieve info. from sites, or in general improve your web experience. Greasemonkey is a very popular extension now.The scripts run by greasemonkey are all java scripts. You can either write your own scripts or download them from a repository of  javascripts  at:

http://userscripts.org/