Firefox has long been criticized for its memory usage, so one of the major focuses for Firefox 3 development has been better memory management.
Here's an excellent article written about what was done, including explanations and benchmarks to illustrate all the awesome work that has gone into memory management for Firefox 3.
Firefox 3 is still in beta, but hopefully it will be released later in 2008. And the changes aren't all under the hood -- the interface has been streamlined and glitzed up. Whereas previous versions of Firefox went for the same look on every platform, the look is now customized per-platform in order to take advantage of the pretty themes we're seeing for Vista, OSX, and Linux.
Enjoy, Firefox users. :-)