Yesterday Mozilla made the change official with the launch of a new release channel, dubbed Aurora, that will serve up a stream of Firefox builds that are less fragile than the nightly builds but not as stable as official pre-release betas. In other words, Firefox Aurora is the equivalent of Chromes cutting-edge Dev and thus it's not really for the average user.
Under the new development regime, the Aurora channel will get new features at regular intervals, but some of them might be disabled if they look like they need more work, then re-inserted into a later cycle after fixes have been applied. Right now those who install the experimental Aurora build can get a taste of Firefox 5, although at this point features like hot links from pinned tabs and social network sharing are not in place yet, and add-ons are unlikely to work.
According to the Mozilla blog, Aurora will be synchronized with the Nightly Build server every 6 weeks. We should note that by default Aurora installs over whatever version of Firefox you're currently running, so if you plan to regularly switch between the stable and experimental builds, you might want to configure them to use separate profiles -- we covered how to do this last year using Firefox 3.6 and Firefox 4 Beta but the same method should apply here.
You can download the Aurora build from the new Firefox Channels page. For more details about the new channel system head over to Mozilla's official blog post or refer to the documentation in the Mozilla Developer Center.