First of all, you must install the Firefox add-on GreaseMonkey from the following link: https://addons.mozilla.org/de/firefox/addon/greasemonkey/. Next, you must restart your PC in order to activate the add-on. On this Page: http://userscripts.org/ , you can find many useful scripts in order to customize and enhance your browser experience. When you have find a script which you are interested in, just click on the install button, confirm the installation and you will find the scripts on the Firefox add-on page. There you can add/remove, activate/deactivate scripts and modify options.
You can find further information’s on how to create and modify scripts on the following Homepage: http://wiki.greasespot.net/Main_Page