From the Testing dept.:
Firefox 3 is based on the Gecko 1.9 Web rendering platform, which has been under development for the past 31 months. Building on the previous release, Gecko 1.9 has more than 12,000 updates including some major re-architecting to provide improved performance, stability, rendering correctness, and code simplification and sustainability. Firefox 3 has been built on top of this new platform resulting in a more secure, easier to use, more personal product with a lot more under the hood to offer website and Firefox add-on developers.

[Improved in Beta 4!] Firefox 3 Beta 4 includes more than 900 enhancements from the previous beta, including drastic improvements to performance and memory usage, as well as fixes for stability, platform enhancements and user interface improvements. Many of these improvements were based on community feedback from the previous beta.

mozilla.com