Windows/Mac/Linux: After the release of Firefox 4, Firefox moved to an accelerated, Chrome-like channel system, starting with the bleeding-edge Aurora version of Firefox 5. Today they've released the Beta version, which will let you take advantage of its performance increases in a more stable environment.
If the stable version of Firefox moved too slowly for you but Aurora is a bit too bleeding-edge, Mozilla has added a beta version in between the two that you can follow. It's a little more polished and stable than the Nightly and Aurora channels, but let you test some of those cool new features before they're released to everyone else. Today marks the release of the Firefox 5 beta, which includes performance and stability improvements as well as a fast channel switcher.
The channel switcher means that you don't have to re-download Firefox if you want to change your version of Firefox — you can do it right from the About Firefox menu. If you're running either the Aurora or Beta channel, just go to Help > About and click the "Change" button next to where it tells you what channel you're running. From there, you can switch to the Aurora, Beta or Stable channel any time you want. Hit the link to read more about the new version.
First Rapid Release Firefox Beta Now Available [Mozilla Blog]