Firefox most definitely. If you asked me standard FF VS Opera, it would be close call. But add Tab Mix Plus addon, Customize google addon and NoScript addon to Firefox, and FF wins hands down by a long shot. Actually, just add the Tab Mix Plus addon and it wins hands down.
Firefox 3 is out now as well, which is a big step up over FF 2. Some addons are not yet fully compatible with the newer browser, but it won't be long.
Cheers Greg
