Currently implementing many new features into Postbox, I carefully read (several times) Mark Surman's recent article on Thunderbird's future. I also read Simon Phipps's report twice. Then the contract offer for a Thunderbird Architect posted by Mozilla must be read too:

... Thunderbird is facing a number of technical challenges, including but not limited to:

• ...
• The possible future deprecation of XUL, its current user interface technology and XPCOM, its current component technology, by Mozilla
• ...

In practice, the last line above means for Thunderbird:

1. rewrite the whole UI and the whole JS layer with it
2. most probably rewrite the whole SMTP/MIME/POP/IMAP/LDAP/... layer
3. most probably have a new Add-on layer or, far worse, no more Add-ons

Well, sorry to say, but that's a bit of a « technical challenge »... So yes, that's indeed a « fork in the road » but let's be serious a second, it's unfortunately this kind of fork; rewriting the app is not a question of if but only a question of when. Unless Thunderbird dies entirely, of course.

Evaluating potential hosts for Thunderbird and a fortiori chosing one seems to me rather difficult without first discussing the XUL/XPCOM-less future of the app, i.e. without having in hands the second milestone delivered by the Thunderbird Architect. First things first. I would also be interested in knowing how many people MoCo will dedicate to the deXULXPCOMification of Firefox, that would allow some extrapolations and some pretty solid requirements (and probably rather insurmountable...) for TB's host.

Last but not least and from a more personal point of view, I feel devastated confronting Mark's article and the Mozilla Manifesto.

http://www.glazman.org/weblog/dotclear/index.php?post/2016/04/26/First-things-first