If you havent heard of it before, it's called Greasemonkey. It basically allows you to customise the way Firefox works with little javascripts.
One good example of this (and my personal favourite) is this one called Greased Lightbox. What it does is, when you click on an image link or a Google Image Search result, instead of the image opening up in a new page, it is overlayed over your current window,
a-like so.