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.
Is Earlybird 64bit ?
No, at least not officially. There are some ‘unofficial’ 64-bit builds out there (much like there is Firefox for Windows).
If you claim there are unofficial win64bit Thunderbird 15 out there than why not tell use the url link that is “out there”.
There seems to be no pressure on the lighting developers to provide a win64bit xpi even though it should not take any programming since its already available for linux.
I haven’t personally tried any so I don’t know. You can do a search for Thunderbird 64-bit and see what turns up.