Greasemonkey is one of the most popular extensions for Firefox, which allows you to create JavaScript scripts and then run them while browsing the selected website. With its help, it is possible not only to change the appearance of a selected website, but also to add new features to it. The extension works on the basis of special scripts created in JavaScript language, which allow the user to modify the appearance and layout of the indicated website and remove or add new functionality to it.

These scripts are launched only when you open the browser defined in the URL settings. Creating new scripts unfortunately requires the user to know the JavaScript language, but he can always use ready-made solutions written by other people. There are many repositories with scripts for Greasemonkey available on the Internet, and one of the most popular is http://userscripts.org/.