Awesome extension Rated 5 out of 5 stars
Greasemonkey is more suited for the technically able people, but it is one of the most powerful extensions for Firefox, in my opinion.
I have used it every day since the early versions and published dozens of script for the sites I use.
This extension is a bit difficult to grasp at first because most of the value comes from scripts that the extension lets you install. But it is well worth your time if you want to make the best of your time browsing in Firefox.