The alpha build of Thunderbird is called Earlybird 8.0a2 and is under progress. It is available for installation with the beta as well as the stable build. 
Officially, news on the Early bird is not yet announced and therefore no change logs available as yet.
Many have already tried to take a peek to look for changes that will appear with the new release by installing and running it. However, there do not appear to be any new features to make it a new version 8. The general conclusion so far, is that the Earlybird is making a debut simply to keep in line with the rapid release cycle that Firefox developers, Mozilla, recently announced.
Thunderbird Earlybird 8.0a2 is most likely a Cosmetic Release
Recently, Mozilla chose to introduce new releases to Thunderbird, its email product, more rapidly so that it could keep pace with Firefox, its browsers’ new features. There is a growing belief within Mozilla that the entire user experience with the internet is due to the unique capability of faster, secure and latest browser features of its browser products.
Therefore, they feel the need to introduce the latest developments faster as soon as features are developed to give that cutting-edge to users on the Firefox browser. This in turn has lead to increasing the pace at which the Thunderbird releases too are being introduced. This in turn has lead to routine maintenance of Thunderbird to be released as a new version called Thunderbird 8. Of course, all this will be validated once, its officially released and an exclusive Earlybird release channel page is available online.
Expectations from Earlybird 8.0.a2
There will of course be the routine bug fixing, some interface tweaks and a definite emphasis on improved performance. As of the latest features available on the previous version too, there is nothing truly notable. However, the developer-community as such, is very keen on introducing certain changes in Thunderbird and are avidly discussing it in detail online.
The first feature, expected to go under the scanner is the Address Book. This is a legacy feature, inherited from the Netscape Communicator, before Thunderbird split into its present format. Developers of the likes of Mike Conley has mentioned in his blog post of changes to be introduced to the Address Book.
The Address Book, it is hoped, will soon get its own tab rather than a window in Thunderbird. Currently, contacts-list appear as separate entries from multiple address books/groups. With a tab-feature, this should be tweaked to ‘merge’ as a single contact, more as a viewing option.
Background synchronizing of address book as well as remaining independent entities is expected if this feature were to be fully developed. Editing information and retaining it or erasing it (do and undo features) should be allowed.
Test Pilot is another feature, which has once again been pushed to a newer release. It was expected on Thunderbird 7 but moved to Thunderbird 8 but is not likely to be ready here as well. Under the present pace, it looks good only for Thunderbird 9 Beta, though it is already available in pre-release versions of Firefox.
Earlybird 8.0a.2 download here.
