CSS版Greasemonkeyのような拡張

Stylish - MozillaZine Forums

What is Stylish?
Stylish is to CSS what Greasemonkey is to JavaScript. Stylish allows you to easily manage user styles for the application UI, all websites, or only certain websites. Stylish is better than using userChrome.css/userContent.css because styles are applied immediately instead of requiring a restart.

リアルタイムに反映されるのはいいですね。さっそくUserContent.cssの中身を移植して、さらにGreasemonkeyに入れていたフォント変更用Scriptも移植。これは便利!