There will now be four channels in total, Aurora being the new one created for experimental releases. As work on Firefox 5 has already started, the first Aurora build, or rather the merger of mozilla-central (Nightly) with the mozilla-aurora channel, has now been released. Being the first ever release, there are some quirks, but it's yours for the taking at this very moment.
Right now, it's nothing more than a copy of a very recent Nightly build. In fact, you'll notice that the very latest Nightly build comes with the new branding and About box, while the Aurora build is still named Minefield and uses the Nightly logo.
However, these small discrepancies should be fixed soon and the Aurora logo and branding will be added in a future update.
The About box is the same from Minefiled as well, except for one important detail, it now has a drop-down menu enabling users to switch between the the three main channels, Aurora, Beta and Release.
This makes it a breeze to switch between channels if you want a more cutting edge version or you need a more stable release, something that Chrome should probably have added long time ago.
At this point though you're not going to see too many differences between Firefox 4 and the Firefox 5 Aurora build, which is labeled as Firefox 4.2a1pre. In fact, it's likely that Firefox 5 will be more about adjusting to the new release schedule and development model rather than adding new features. If all goes according to plan, we should be seeing Firefox 5 final land in about three months.