Greasemonkey is a Firefox extension that allows you to customize the way webpages look and function. Hundreds of scripts are already available for free. And if youre the tinkerer sort, you can also write your own.
Styles and skins for all popular sites in the web FreeStyler is a browser add-on that has everything you might need to style up websites you visit according to your taste. It provides a great number of ready-to-use styles with convenient system of managing them.