I the unlikely event that someone reads this post, is a web developer, and hasn't heard of Firebug, you need to keep going...
FireBug is the ultamate developer extension for firefox. It essentially allows you to walk the DOM tree in a nice way. But when I say nice way, I mean very nice way. It's just utterly cool. I find new features and find I can do new things with it everyday. I can't imagine doing CSS, javascript or web development without it.
If you don't don't have it, go, go get it