Aardvark is a Firefox extension that I frequently use before resorting to MODIv2. Both of these tools outline elements in the browser and reveal the underlying CSS. When CSS is being mystical and you have keyboard shaped dents in your forehead, these tools can pull the man out from behind the curtain and help bring on that a-ha! moment which solve the problem. Aardvark needs to be installed from the developer’s site as its review for addition in the Mozilla Firefox Add-ons is not complete. I highly recommend Aardvark!
Oh, the Web Developer Toolbar also has similar functionality under Information->Display Element Information. (cntl+shift+f) The Web Developer Toolbar is the absolute most essential Firefox add-on for us Internet junkies!
Update: Like MODIv2, Aardvark now comes as a bookmarklet! That means Aardvark can be used with Internet Explorer and Safari also.