Greasemonkey is a Firefox extension that allows you to write scripts that alter the web pages you visit. You can use it to make a web site more readable or more usable. You can fix rendering bugs that the site owner can’t be bothered to fix themselves. You can alter pages so they work better with assisting technologies that speak a web page out loud or convert it to Braille. You can even automatically retrieve data from other sites to make two sites more interconnected.
Greasemonkey by itself does none of these things. In fact, after you install it, you won’t notice any change at all… until you start installing what are called “user scripts”. A user script is just a chunk of JavaScript code, with some additional information that tells Greasemonkey where and when it should be run. Each user script can target a specific page, a specific site, or a group of sites. A user script can do anything you can do in JavaScript. In fact, it can do even more than that, because Greasemonkey provides special functions that are only available to user scripts. There is a Greasemonkey script repository that contains hundreds of user scripts that people have written to scratch their own personal itches. Once you write your own user script, you can add it to the repository if you think others might find it useful. Or you can keep it to yourself, content in the knowledge that you’ve made your own browsing experience is a little better. There is also a Greasemonkey mailing list, where you can ask questions, announce user scripts, and discuss ideas for new features. The Greasemonkey developers frequent the list; they may even answer your question! Check out http://wiki.greasespot.net/ to get started. Works with: 
Firefox: 1.5 – 3.0b5pre
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