Firefox - Web Developer extension

Web Developer extension,includes some EditCSS functionality:
The Web Developer extension adds a menu and a toolbar to the browser with various web developer tools. It is designed for Firefox, Flock, Mozilla and Seamonkey, and will run on any platform that these browsers support including Windows, Mac OS X and Linux.

Use the Web Developer Toolbar to Locate Style Rules
The Web Developer extension places a number of menus in a new toolbar. There are far too many features to go into all of them here, so we'll stick to those relevant to this hack. However, do make sure you look at all of them.

One of the most time-consuming aspects of debugging and tweaking CSS is working out which CSS statements are causing the problem. More than one CSS statement can affect the same HTML element, and elements inherit CSS properties from their ancestors. The Web Developer toolbar helps isolate the relationship between a style sheet and an HTML document.

Download the : Web Developer extension