Screaming Penguin - Source Does Matter

And, it uses less memory than IE and Opera as well.

Firefox 3 has gotten a lot of focused effort on cutting the footprint and improving the performance. As Ryan Paul at Artima notes the work has paid off - FF3b4 takes up a lot less memory and is faster.

Also interesting is that the Mozilla team is even talking about taking FF to mobile environments, something the Ars story also touches on:

What it shows to anyone who looks is that we're able to hit the kinds of memory and performance requirements that mobile platforms demand, . . .