Grab Mozilla/Firefox here:
http://www.mozilla.org/

The Mozilla/Firefox WebDeveloper extension can be found here:
http://extensionroom.mozdev.org/more-info/webdeveloper

For more info on what it does:
http://www.chrispederick.com/work/firefox/webdeveloper/features/

For a really good selection of other extensions (different sites showing same/similiar extensions in case one is down):
http://update.mozilla.org/extensions/?application=firefox
http://extensionroom.mozdev.org/
http://texturizer.net/firefox/extensions/

Truth be told, if I didn't have the above extension I'd still be totally lost with theme modifications. I learned more about css from it than any book or class could have taught me. Just knowing what classes were applied to what elements in the theme while looking at the theme makes a world of a difference.

No affiliation with Moz, blah blah blah. I'm just a pretty darn happy user.

Thanks Speed :)
I already had Mozilla, i just didn't know about that extension.

I'd happily ditch IE altogether except as a theme designer I need to be able to view themes in a browser 90% of my site visitors use :(