By Stylish is a Firefox, Thunderbird, Flock, SeaMonkey, Mozilla Suite, and Songbird 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 dont even need to know how to write styles yourself; just a couple clicks and the chosen style is applied. Stylish is to CSS what Greasemonkey is to JavaScript, and unlike other methods of using user styles, most styles take effect immediately.

New in 0.5
Updated user interface
Filtering of installed styles
Improved CSS editing tools
Support for SeaMonkey, Thunderbird, Mozilla Suite, and Thunderbird
Known issues
You can turn on and then immediately turn off a style, but if you load a page after the application of a style, you cant instant unapply it.
Instant apply has a slightly different priority than normal apply. As long as you use !important, it shouldnt be a problem.
Some CSS properties dont instant apply well on some elements. This is especially true of XUL elements. Id be interested in knowing what exactly doesnt work so I can file bugs on the problems.
For some platforms/themes, checkboxes may not be too visible in the Manage Styles window when the style is highlighted.
@import statements pointing to unavailable servers will hang the application. Its strongly recommended you not install styles with @import statements (such styles are not allowed at userstyles.org)