Released: March 10, 2008

Firefox 3 Beta 4 is a developer preview release of Mozilla’s next generation Firefox browser and is being made available for testing purposes only.

[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.

http://www.mozilla.com/en-US/firefox/3.0b4/releasenotes/#download