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. ... 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.
If you are running Firefox, Help → Check for Updates... Or download. Please note that this does break a lot of plugins.