Firefox does a much better job at cleaning up after itself
Firefox 14 promises quite a large drop in memory usage, particularly after longer sessions. Mozilla has been working on improving Firefox memory usage for the past several releases. The Memshrink team has been keeping track of progress and provides the "Are we slim yet?" website which illustrates all of the improvements with actual recorded data.While memory usage has been dropping gradually for a few releases now, Firefox 14 provides a major improvement to releasing memory, i.e. after all tabs have been closed.
In a perfect world, closing a tab would free up the memory used by that tab, but that doesn't always happen. While plenty of people complain of leaks and big memory usage, most of the time due to add-ons rather than the browser, the improvements Firefox has been seeing are impressive.
Memory usage may not really mean much on desktops, unless you keep your browser running for many hours, open hundreds of tabs and so on, but it's very important on mobile devices.