It's a public preview of the new Firefox code, available in 45 languages, aimed at developers and early adopters to test out the new features. It has an extensive list of known bugs.
Among the major improvements are changes in the look and feel on the various operating systems, major security enhancements, and increased performance and stability. It is based on the Gecko 1.9 rendering engine, which includes 14,000 updates to the code.
On the performance front, the documentation states that applications such as Google Mail and Zoho Office run twice as fast in Firefox 3, compared to Firefox 2, and memory usage has been improved. In addition, bookmarks, history, cookies, and preferences are less susceptible to data loss.
A list of all the new features is available on the Mozilla Firefox site.