If you develop and trouble-shoot a lot of web sites that use CSS, then there are two Firefox Add-on plug-ins that you have to use and will ultimately “CANNOT” live without.
by Chris Pederick
Adds a menu and a toolbar with various web developer tools such as finding broken images, resizing browser, colour info, etc.
by Joe Hewitt
Oh…for all those times where a web site uses multiple, multiple CSS files and you can’t figure out which CSS element is being used and which CSS file it is coming from. Just hover over an element and Firebug will show you which CSS element is being used.