I only noticed this new Firefox Extension today. It's 1.0 version was released yesterday only. You install it, add it to your toolbar and then you can use it to inspect elements to get the complete rundown of the CSS declaration for a chosen element.
I've haven't tested it fully yet but it looks really promising. If you're a web developer, this is a must. It's much better than the Firebug CSS inspector but someone more clever than me can probably point out that they serve different purposes.