Greasemonkey and Firefox
You can download Greasemonkey from its page on Firefox add-ons.
Once you have it installed, go to userscripts.org to find some scripts. There’s no telling what you might find. To install a script, click on a link to the script’s .js file, and Greasemonkey will detect it automatically and let you install it.
If you need additional help, visit the official Greasemonkey website.