The idea behind Greasemonkey is pretty simple. It’s a Firefox extension, installed in the same way as any other Firefox extension (find it via the Tools > Addons menu and hit Install).
However, it doesn’t do anything in and of itself: what it does is to enable you to run scripts, either by other people or by yourself, which will alter the way web pages look and function.
Very Easy Guide to use Grease Monkey
See on www.techradar.com