Greasemonkey is a Firefox extension which lets you to add bits of DHTML ("user scripts") to any webpage to change it's behavior. In much the same way that user CSS lets you take control of a webpage's style, user scripts let you easily control any aspect of a webpage's design or interaction.
It's kind of a more organized approach to site-specific bookmarklets. I've done some right-click context menu extensions in IE to work around problems / annoyances in intranet apps before; this would be perfect for that kind of thing.
There are already some useful scripts available:
- Make sure that all URLs displayed in the browser are clickable links
- Improve the usability of a site you frequent (AllMusic)
- Route around common and annoying website bugs
- KillBlank - converts target="_blank" links to use current window