Greasemonkey

Greasemonkey is a Mozilla Firefox extension that allows users to install scripts that make on-the-fly changes to web page content after or before the page is loaded in the browser (also known as augmented browsing).
The changes made to the web pages are executed every time the page is viewed, making them effectively permanent for the user running the script.
Greasemonkey can be used for customizing page appearance, adding new functions to web pages (for example, embedding price comparisons within shopping sites), fixing rendering bugs, combining data from multiple web pages, and numerous other purposes.
The Greasemonkey project began 28 November 2004.
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….