Stylish is an extension for Firefox/Flock and Thunderbird by Jason Barnabe (np) that lets you manage CSS rules to change the appearance of web sites. It can even change the appearance of the application (Firefox/Flock/Thunderbird) itself.
I’ve been using Stylish since its first released. Previously, I used Custom User Style Editor — a module in Shimoda Hiroshi‘s Context Menu Extensions — and Cskinner — a Greasemonkey script by Matt Labrum — to change the appearance of web sites.
Stylish is also best for
blocking hiding ads on the web sites. It works better than Greasemonkey or RIP, because Stylish hides ads even before the page is fully loaded. After using Stylish for about two months, I removed RIP permanently.