Christian Legnitto, Mozilla's Engineering Project Manager, has detailed the release schedule for Firefox 5, 6 and 7. Unless major issues are discovered with the scheme, this is how builds in the assorted repositories will identify themselves:
"Firefox 6.0a1 will be released next week on April 12, Firefox 7.0a1 in the mid-of May. The final build of Firefox 5 is expected on an around June 21."
Because updates are (intended) to be silent and automatic we can generally assume a user on a given channel is on the latest available build. This assumption simplifies things greatly.
Along with the faster 6-week release cadence, Firefox's new Chrome-like release channels have also been given names and anticipated update frequencies. The most notable change is the introduction of a new alpha channel, which's analogous to Chrome Canary called as "aurora" and will update nightly. Aurora will be where fixes and features are tested, and either approved for Beta, or backed out to Central. Aurora will have a new icon, too.
The Nightly (or mozilla-central) channel will remain unchanged in name and frequency, and will gain a new "nightly icon." The Beta (mozilla-beta) channel will remain as-is, with new builds rolling out weekly. The Release (mozilla-release) channel will also remain as-is, with security and stability updates coming every 6 to 12 weeks.