Well, I've managed to screw up both the Mac and my Windows Vista machine's Firefox browser. The Mac requires that I upgrade the OS to Leopard - which will happen in January, probably. I'm not sure what happened to the Vista machine, but the version of Firefox 3 that I downloaded won't tab properly and I can't find a way to solve the problem - but I haven't tried very hard.
So, on the Vista machine I'm using Google Chrome. It works well and has a couple of nice features. Faster than Firefox when it loads and less nagging when it encounters pages it doesn't like than Explorer.
One feature I particularly like is the page you see when you click on the new tab. On that page you see images of the pages you most recently visited. I haven't figured out how many you'll get to see, since I haven't been using it that long. I'm sure I will find out soon.
On the Mac I've switched to Opera. Haven't done much on it, so I don't have much to report. Like Google Chrome it loads more quickly than Firefox - or Safari (a slow loading browser). Beyond that, no comment.