As we all know, the previous versions of Firefox weren't very memory efficient; sometimes the browser would use more than 400MB after few hours if you visited script and image intensive pages and used multiple tabs.
This bug was reported by many users to Mozilla, but they didn't fix it until now. In 2006 they made a historical statement that a memory leak is not a bug, but a feature. You can check out the original blog post here.
It seems that Mozilla managed to fix Firefox's nasty habit to hog memory, and according to benchmarks, Firefox 3 is the most efficient browser on the market. Compared to Flock 1.2, which is based on Firefox 2, Firefox 3 has 50% better results and memory usage almost never goes over 120MB.
Opera 9.50 performed better than Internet Explorer 8 Beta 1, and the worst performer on the test was Safari, which at end of the test used over 600MB. The tests were conducted after more than two hours of work under Windows and memory usage was recorded every 3 seconds, so we can now see from real-world usage how Firefox 3 compared to other browsers on the market.
You can find graphs and more information here.