Windows/Mac/Linux: Mozilla today launched Aurora, a semi-stable dev build of Firefox that represents Mozilla's move toward a new, rapid-release cycle aimed at "giv[ing]users more opportunities to participate in building Firefox".
Mozilla's always had nightly (Minefield) and beta builds that users could test drive, but Aurora adds a release that lives between the nightly build (now simply called Nightly) and the beta build. Aurora is a bleeding edge release that updates every six weeks, but it also undergoes some QA before updating, making Aurora more stable than Nightly — and a browser that power users might realistically use as their default.
Most Google Chrome power users run the beta or dev channels of the Chrome browser because it offers access to new features sooner, and Google generally does a good job of keeping this "unstable" channel relatively stable. Hopefully Aurora provides Firefox users with an in-road to do the same thing. Mozilla explains their thinking on the Mozilla Blog, or in more detail at the Mozilla Developer Center. You can download your channel of choice at the link below.
Firefox Future Releases [Mozilla]