Stylish is a Firefox extension that allows easy management of user styles.
User styles empower your browsing experience by letting you fix ugly sites, customize the look of your browser or mail client, or just have fun.
With an online repository at userstyles.org, you don't even need to know how to write styles yourself; just a couple clicks and the chosen style is applied.
Having both JSView and Stylish can cause crashes in Firefox 2.0.
If you're experiencing crashing opening the Manage dialog, in about:config set extensions.stylish.fileVersion to 2 and try again.