"Most Firefox users are familiar with the Greasemonkey
extension, a powerful tool that lets you enhance your browsing
side. The Stylish extension gives you that same level of control
page's CSS cannot change its functionality--but it can greatly
enrich its usability.
"Stylish builds on Firefox's User Styles feature, which lets you
override the CSS rules of any or all pages you visit. You can
manually add CSS rules to Firefox's userContent.css file, including
assigning specific rules to URLs that match a given pattern.
Stylish provides you with a simple point-and-click way to do the
same thing, and a nice interface with which to keep track of the
styles you have selected..."