Stuart Parmenter, a Mozilla engineer, has some interesting news about memory usage in Firefox 3 Beta 4:

We’ve made more significant changes to the platform than I can count, including many to reduce our memory footprint. The result has been dramatic, and you can see for yourself by getting a copy of the recently released Firefox 3 Beta 4.

Here’s What We’ve Done:

Reduced Memory fragmentation
Fixed cycles with the Cycle collector
Tuned our caches
Adjusted how we store image data
Hunted down leaks

Our work has paid off.

We’re significantly smaller than previous versions of Firefox and other browsers. You can keep the browser open for much longer using much less memory. Extensions are much less likely to cause leaks. We’ve got automated tools in place to detect leaks that might result from new code. We’re always monitoring and testing to make sure we’re moving in the right direction. All of this has been done while dramatically improving performance.

So this is all very interesting, and you should read the full post for the details of how they did it. But even after just a few days of use, it's clear that Firefox 3 Beta 4 is using tons less memory than FF2. I used to leave FF2 on overnight and the thing would gobble up 300+ MB of RAM easily. I check it from time to time and it's usually in the 60-70 MB RAM range, a huge improvement.

If they can really put this behind them, I'd like to see the same dedication go towards making a high quality FF user experience. The new UI in FF3 is simply horrible, especially on Windows Vista. It shouldn't be this amateurish.