Get it now from Mozdev. You will not be disappointed.
I’ll let Mozdev explain what it does:
In case you aren’t a techie, that boils down to this: You can execute arbitrary JavaScript to modify any webpage any way you want. This is especially useful in removing ads even AdBlock can’t touch, but you can also make websites more useful in addition to simply being less annoying. A case in point is the Google Butler, which includes links to other search sites and alters which font is used to display the page.
This is a great webpage for user scripts, the extensions that Greasemonkey hooks into the browser itself. It’s a long list broken down by which website the user script is aimed at, although many of them are general-purpose.
Dive into Greasemonkey isn’t the best introductory work I’ve ever read, but it does help you get your feet wet in writing your own user scripts. It is freely downloadable in HTML and PDF.
If you do want to write your own extensions, you will want this JavaScript shell (implemented as an active bookmark written in JavaScript, a fact cool in and of itself). It actually gives you access to all of the bits on a webpage JavaScript programs can touch. It’s invaluable if you are using this as an excuse to learn JavaScript as well as extension-writing.
(Speaking of extensions, you can compile Greasemonkey user scripts to standalone Firefox extensions. Which is nice because it’s a lot easier to write a user script than a full extension.)
Greasemonkey is generating buzz. Some people dislike the level of control it gives users, apparently, but I think it’s long overdue.
So try it out and have fun.