Grease you monkey
Greasemonkey is one of the most powerful features of Firefox. It allows you to attach small javascript files to existing Websites to change them in ways that make them more useful to the user.
I've used them in special ocasions but Firefox is not my main browser (that task goes to Safari), so I'm not a regular user.
If you want to see a nice example (with screencast) of a Greasemonkey script, look no further than this.
