Greasemonkey is a Firefox add-on which uses small chunks of JavaScript to customise how a web page works.

You don't have to know anything about JavaScript to use Greasemonkey, as there are plenty of free scripts available. And it's surprising just what they can do. As we write, one repository (Greasy Fork) has scripts to avoid ads, extend YouTube, bulk-delete items from your Facebook timeline, download videos, translate text, check Rapidshare links, download Instagram and Facebook albums, and automate, fix and enhance just about every aspect of Kings of Camelot (and that's just on the first three pages).

These scripts can be almost as powerful as add-ons, and they're usually just as easy to install. Find something you like at Greasy Fork, click the link to read more, and if you like the sound of it, click "Install this script". A warning dialog explains what you're about to do, so there's no risk of silent installations. There's an option to view the script source, but otherwise you can install it with a click. Installed scripts can then be accessed via an extra tab on the Addons page, where you'll find buttons to set options, disable or remove them.

Greasemonkey also provides simple tools to create and manage your own scripts (click the Greasemonkey button on the address bar and choose "New User Script"). Even experts may find it a challenge to get everything working, but fortunately Greasemonkey has a comprehensive wiki to offer help and advice.

Version 3.9 changes (News):

Feature enhancements:
 

Improve the performance of GM_getValue/GM_setValue. (#2197, #2025)
Add a rich tooltip to the Greasemonkey toolbar button.  (#2377, #2418)
GM_getResourceText supports a second (responseType) parameter, for better compatibility with binary files. (#2376)

Fixed bugs:
 

Global exclude rules no longer require a Firefox restart to take effect. (#2410)
A web page whose url ends in ".user.js" could fail to load. (#2407)
A script whose name ends in ".db" could fail. (#2402)

Verdict

Greasemonkey extends Firefox by providing easy access to a huge range of scripts. Beginners can use it without worrying about the fine details, but if you're a JavaScript expert then it also gives you plenty of site-customising power.