For a long time, we've known that we've outgrown the current version of the AUS software. Recently we've been working hard on a drop-in replacement for it - something that would give us easier and better controls over the updates we serve. This new software, codenamed Balrog, is nearly ready to start serving updates in production. We're still bringing the last parts of the new system online, but we intend to make this change before the end of September. We will announce an exact date at least a day before the transition.
Of course, accurately serving updates at scale is very important to Mozilla, so even after these preparations we'll be rolling out carefully. We've already tested Balrog internally for many months without issue, and we'll start by switching Nightly and Nightly-based (UX, elm, oak) users of Firefox and Fennec over. Aurora, Beta, Release and ESR builds will continue talking to the old AUS until we're confident enough to start moving them over. Thunderbird users will also be moved when we feel comfortable.
While we're confident that the transition will go smoothly, we ask that anyone who experiences problems after the switch to file a bug or look for bhearsum or nthomas in #releng.