http://www.getfirebug.com/ I've only just begun to use it, but it's seriously good. One great feature is the 'inspect' functionality; you click on an element and Firebug displays all the relevant CSS, in order of specificity I believe.
OK you've convinced me to give it another try. I had this in a while ago but took it out with a couple of other extensions when I had a bit of a clearout because things seemed to be getting slow. Of course you've already got the "Web Developer" toolbar? (btw that "html validator" you recommended is really useful- keeps tabs on your code as you write it.)
Yeah - I'd say this is one that's worth keeping. I actually need to reinstall the Web Developer Toolbar as I freshly installed Firefox 2 - thanks for the reminder.
I have run both for a while now with no problems - in fact it has helped me greatly while trying to get some Javascript running. (Had to learn that for an AJAX site!)
