GreaseMonkey

GreaseMonkey is perhaps the coolest Firefox extension I've found yet. You can do all kinds of cool web page modifications with it, as listed here. Right now, I have all links to New York Times articles going to the single-page format, and a script that fixes some of the crappiness in allmusic. Once you've installed the extension, to install the linked scripts just right-click and select "Install User Script...".