Mozilla released Firefox 4.0 Beta 7, you can download it over here.
According to many reports on the web, this new version is pretty speedy:
During our tests of the new beta, we were consistently impressed by its outstanding performance and greatly improved responsiveness. It delivers highly competitive performance and puts Firefox back on an even footing with its rivals. The beta also brings the Firefox 4 theming overhaul to Linux, including the new tab position above the address bar.