OStatic

Greasemonkey is a Mozilla Firefox extension that allows users to install scripts that make on-the-fly changes to most HTML-based web pages. 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 functionality to web pages (for example, embedding price comparison in Amazon.com web pages), fixing rendering bugs, combining data from multiple webpages, and numerous other purposes. Well-written Greasemonkey scripts can integrate changes so well that their additions appear to be natural parts of the web page.

Most Greasemonkey user scripts are written by hand, using site-specific JavaScript code which manipulates the contents of a web page using the Document Object Model interface. userscripts.org maintains a database of Greasemonkey scripts, and for each it lists the URLs of web pages to which the scripts pertain. (N.B. The obsolescent script repository might contain some older scripts that have not yet migrated to the new repository.) When the user loads a matching page, Greasemonkey invokes the relevant scripts, which can then add to the page, delete parts of it, or move parts around. Greasemonkey scripts can also poll external HTTP resources via a non-domain-restricted XMLHTTP request.