I love Firefox and its addons, although I still habitually use Internet Explorer. IE6 at work (which always provides some nice CSS challenges!) and IE7 at home.
I'm a bit slow off the mark with using firefox, but the latest addon that someone sent me yesterday may have tipped the balance.
The IE Tab addon actually uses the IE rendering engine to render in a new tab in Firefox. It's quite amusing when you right-click and get the IE context menu.
The really great thing about this is that you can then use the power of the other Firefox addons to develop for IE. Here's my list of 'must haves'.
The IE Developer Toolbar is still a pretty good tool, but Firebug just gives you more info when you're debugging CSS (which is inevitable with IE).