Initially the memory consumption of Firefox 3 looked higher but further testing revealed the new browser doesn't suffer from spiraling memory consumption when the browser is under heavy load. Memory usage after opening 12 pages into the browser and wait 5 minutes:
Unfortunately they didn't include Opera.Firefox 2.0.0.9: 103,180KB Firefox 3.0 beta 1: 62,312KB Internet Explorer 7: 89,756KB