You’re probably not use the Chickenfoot Firefox extension. You really should be. It’s basically an extremely cool API and plugin for writing user scripts (similar to Greasemonkey, only awesome), with very clever pattern matching for finding the relevant elements on the page, which has always been the most annoying thing with writing user scripts.
I originally found it via digging through google techtalks. The original techtalk is here (it’s quite long).
Also, here’s a useful snippet for making it less annoying to use from the keyboard. It’s not mine – the first part is due to Rob Miller and the second is a trivial adaption of the first. It is however really rather handy.