Greasemonkey is a Firefox extension that allows you to customize the way webpages look and function.
Greasemonkey is a Mozilla Firefox extension that allows users to install scripts that make on-the-fly changes to HTML web page content on the DOMContentLoaded event, which happens immediately after it is loaded in the browser (also known as augmented browsing).
As Greasemonkey scripts are persistent, the changes made to the web pages are executed every time the page is opened, making them effectively permanent for the user running the script.
Greasemonkey can be used for adding new functions to web pages (for example, embedding price comparisons within shopping sites), fixing rendering bugs, combining data from multiple webpages, and numerous other purposes.
Hundreds of scripts are already available for free. And if you’re the tinkerer sort, you can also write your own. Available at http://userscripts.org
Greasemonkey user scripts are written in JavaScript and manipulate the contents of a web page using the Document Object Model interface. Scripts are generally written to be either page-specific or domain-specific (applying to all pages within a domain) but may also be tagged to apply to all domains for global browser enhancements. userscripts.org maintains a database of Greasemonkey scripts, and for each, lists the URLs of web pages to which the script pertains. Users of Greasemonkey can write or download scripts and save them to their own personal library. When users visit a website matching a script in their personal script library, Greasemonkey invokes the relevant scripts, which can modify a webpage in any way which JavaScript allows — with certain Greasemonkey security restrictions. Greasemonkey scripts can also poll external HTTP resources via a non-domain-restricted XMLHTTP request, allowing external content to be merged with the original page content. Scripts are named somename.user.js, and Greasemonkey offers to install any such script when a URL ending in that suffix is requested. Greasemonkey scripts contain optional metadata, which specifies the name of the script, a description, relevant resources to the script, a namespace URL used to differentiate identically named scripts, and URL patterns for which the script is intended to be invoked or not.
Writing a Greasemonkey script is similar to writing JavaScript for a web page, with some additional allowances such as cross-site XMLHttpRequests. Compared to writing a full-fledged Firefox extension, user scripting is a very modest step up in complexity from basic web programming. However, Greasemonkey scripts are limited due to security restrictions imposed by Mozilla’s XPCNativeWrappers. For example, Greasemonkey scripts do not have access to many of Firefox’s components, such as the download manager, I/O processes or its main toolbars. Additionally, Greasemonkey scripts run per instance of a matching webpage. Because of this, managing lists of items globally is difficult. However, script writers have been using cookies and Greasemonkey even offers APIs such as GM_getValue and GM_setValue to overcome this.
Bug fixes in this release:

• GM_openInTab() fails in Firefox 3. (#1361)
• Firefox 5: Error in Add-ons Manager. (#1367)

Enhancements:

• Add “get user scripts” link to the Add-ons Manager. (#1213)
• Pass site cookies when downloading scripts. (#1359)