greasemonkey
I had been reading about Greasemonkey for some time now, but I had not tried it. I did, a couple of days ago -hey this thing is so cool!
GreaseMonkey is a Firefox plugin that allows you use "user scripts" on a per-url basis. A "user script" is nothing more than a javascript file, but... hey! you can use javascript to completely change the way a web page looks, add links and functionality, rewrite links... Oh, I'm really into this dirty monkey!
Here are a couple of useful links:
I'm sure I'll be back with my own user scripts soon!