Think waaaaayyyyy back...
IE6 was a badly-written, compatibility-breaking, resource-hogging, security-bug written pile of fetid garbage that MS had pretty much stopped developing entirely. Firefox became popular to fight against that scourge. While subsequent versions of IE (when they finally came out) were not entirely great, they represented a significant step forward that realized what made Firefox so popular.
If IE 7 had been out at the time Firefox was released, I doubt Firefox ever would have become particularly popular. And the version of IE in the works discards MS's sordid standards-breaking legacy entirely, and will be no more broken, standards-wise than the other major browsers.
All I have to say about the memory leaks is that Chrome has never "locked" my hard drive light on for several minutes upon closing it to clean up the multiple GB of memory it decided to consume. The one-process-per-tab architecture of Chrome has real advantages, the biggest being when a tab leaks like a sieve (and this doesn't happen very often), you don't have to close every browser instance to clean it up.