When I do web development, my browser of choice is Firefox. With geat extensions like Web Developer, tracking down HTML / CSS issues is made so much easier. There are plenty of others like Live HTTP Headers and Search Status to name just 2.
However when I am just browsing the web, my browser of choice is Opera, and has been for a while now. Performance wise I have found it better than firefox. This is just a perception, without any rigorous analytics. part of the answer is probably to do with FF being slowed down by the installed extensions. Remember I am running Linux (Ubuntu), so Safari and Google Chrome are not available at this time. Konqueror is a good alternative when I was on KDE.
Yes Opera is not free software, just freeware, but for me in this case, its the best tool for the job.