Greasemonkey is a great Firefox extension that you can find here.
It lets you add scripts to modify web pages in place without restarting the browser and without needing to know anything about writing a Firefox extension.
Here are some that I've written or modified. Disclaimer: I can't guarantee they won't accidentally mess something up, and you shouldn't trust random scripts from the Internet without understanding them.
- Bug Me Not - On websites that require registration, optionally try to retrieve one from bugmenot.com
- PRE tag wrapper - Converts preformatted elements to properly escaped typewriter-text elements, so the text wraps correctly without having horizontal scrollbars