GreaseMonkey is a firefox extension that you will need to add user scripts. User scripts allow users to alter the content and behavior of any website through scripts which work inside the browser. There’s a repository of useful user scripts that implement customisations to certain sites, from showing prices inclusive of tax on EBay UK to fixing IE-specific bits in the MSDN documentation. I have used greasemonkey to customize my gmail account. I wanted to have “Delete” button so I used this userscript from “GreaseMonkeyUserScripts”.

Its called Smart Delete Button because it enables/disables itself unlike other gmail buttons.

There are plenty other user scripts for many sites which are interesting and can make your browsing easier. Check them out and let me know which ones you like.