Earlybird is Mozilla’s alpha build of Thunderbird. It’s the Thunderbird equivalent of Firefox Aurora, and like Aurora, installs alongside an existing stable or beta build of Thunderbird, allowing you to run this early developmental version side-by-side with your existing version.
The reason is clear: as an alpha build, Earlybird represents an extremely early developmental version of Thunderbird, with many bugs and glitches still to be discovered and ironed out. However, while you can run it alongside Thunderbird, it does share the same settings and data files, which means you should back up your email (using MozBackup) when using it.
We’d also recommend inexperienced users steer well away from Earlybird, and stick to the Final release instead.
One of the more interesting consequences of using Earlybird is that you’ll be exposed to features that might not yet make it to the beta or Final release stage. Once Mozilla creates an Aurora build, it locks in the feature set for that build, and they’re then tested over a six-week period before the build moves to Beta.
It’s possible that if certain features prove too troublesome during that testing period that they’ll be held back to a future release, meaning you could find yourself using features that may not appear in the subsequent Beta version if they can’t be made stable enough for the next phase of development.
The current version of Earlybird is at 39.0a2.