After a little over a week delay, the merge for Thunderbird 15 on to the Earlybird channel is completed. However, at this time the update channel has not been reactivated. If you want to move to Thunderbird 15 Earlybird you can do so by manually downloading and installing Earlybird. Users can keep checking for an update via Help > About Earlybird.
Warning: Lightning Users! There are currently no builds available for Windows or Mac OSX on either the Earlybird or Daily builds. Keep checking the MDN Calendar Versions site to check for updates.