For designers, the best tool to use when designing a web page is a dynamic CSS add-on that takes your existing CSS, modifies it, and views your changes immediately. Using something similar to Web Developer's edit CSS option is fantastic for a one-size-fits-all add-on, but if you want something a little more slim and trim, i would recommend CSSMate.
After installation, CSSMate is activated through either the View/Sidebar/CSSMate pulldown menu or by using the shortcut keystroke of Ctrl-Shift-8. The sidebar opens on the left and loads all of the CSS files associated with the web page you are currently on. As soon as you make a change to the CSS, the web page is instantly refreshed with your changes.
According to the author, 90% of the extension was gutted and completely rewritten. I haven't ran the previous version, but you can tell when a version of a piece of software is coded properly.
Overall, the extension is a solid 5 out of 5.