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.
Firebug is most likely the culprit [1]. Firefox 15 will have a general fix for the kind of memory leaks such as the one caused by Firebug (affectionately called "Hueyfix" after its inventor Kyle Huey by some) [2].
