Firefox 13, when not overwhelmed by add-ons is the memory champion right now. Seriously, check it out. Download Firefox 13, create a new profile clean from crapware, load up a couple dozen tabs and surf around for a day. Do the same in IE and Chrome. Firefox wins. Firefox 15, just around the corner, has world-beating memory usage and it uses less RAM than any other browser, even when you've got "leaky extensions" installed because we've prevented extensions from causing Firefox leaks.