What does Greasemonkey do?
Greasemonkey lets you add JavaScript code (called “user scripts”) to any web page, which will run when its HTML code has loaded. Compared to writing extensions, user scripts often offer a light-weight alternative, requiring no browser restart on user script installation nor removal, and work with the common DOM API familiar to any web developer (with somewhat elevated privileges for doing cross domain XMLHttpRequest requests and storing small portions of private data). User scripts work more or less like bookmarklets automatically invoked for any URLs matching one or more glob patterns.
