Mozilla on Thursday released the third beta version of Firefox 3.1, aka Shiretoko, one of the frontrunners in the current race to improve Web browsers. According to the Firefox 3.1b3 release notes, the new version includes better “Web worker” multitasking abilities, a faster Gecko rendering engine for showing Web pages, and upgrades to the TraceMonkey engine for faster, more stable execution of Web sites’ JavaScript programs.

Firefox is vying with Apple’s Safari 4 beta and Google’s Chrome for the best JavaScript performance, a factor that’s important for the new generation of sophisticated Web sites such as Google Docs and or Facebook applications. So far, Internet Explorer has the dominant market share, with Firefox in second place.

Mozilla earlier expected the third beta version to be the final beta, but those plans changed. Because of the magnitude of the changes in the 3.1 betas, the next full release will be numbered 3.5. A fourth beta, 3.5b4 is planned, too, followed by the release-candidate cycle, before the final version 3.5 is released, Mozilla said.