User­scripts can make on-the-fly changes to HTML web pages on the DOM­Con­tent­Loaded event. As these scripts are per­sis­tent, the changes made to the web pages are exe­cuted evey time the page is opened. In short, the changes are effec­tively per­ma­nent for the user run­ning the script. On Fire­fox these scripts are installed and man­aged by the Grease­mon­key extension.

Grease­mon­key scripts are writ­ten in JavaScript and can be used for adding new func­tions to web pages, fix­ing bugs, com­bin­ing data from mul­ti­ple web­pages, fill­ing forms, remov­ing con­tent, chang­ing styles, and numer­ous other pur­poses. Userscripts.org main­tains a huge data­base of Grease­mon­key scripts.

On the right you can see my scripts as well as some of my favorites.