One of the biggest “improvements” that Mozilla claims has made its way into Firefox 3 is improved memory usage, in particular, the vanquishing of memory leaks:
Memory usage: Several new technologies work together to reduce the amount of memory used by Firefox 3 over a web browsing session. Memory cycles are broken and collected by an automated cycle collector, a new memory allocator reduces fragmentation, hundreds of leaks have been fixed, and caching strategies have been tuned.
We’re sorry to have to break it to you, but if you thought it was too good to be true you were right. Firefox still uses a lot of memory – way too much memory for a web browser.