I'm still using Firefox. The high memory usage doesn't really bother me because I've got plenty of memory on all my machines. This obviously isn't ideal and it would be nice to have it consume less memory but it doesn't directly impact me so I've just ignored it. I've not noticed it using much CPU though.

Apart from that, I'm happy with how it works, the way the UI looks and the extensions available. I use Adblock Plus, Image Zoom and Web Developer. At one point I did have Greasemonkey running but when I upgraded to Windows 7, I never bothered to put it back on and didn't really miss the tweaks.