Folks seem to have missed half of Mitchell's recent email call to action. It wasn't only about finding a new home for Thunderbird.
Mitchell also says that "we would also like to find contributors committed to creating and implementing a new vision of mail" and that "Mozilla has a range of resources - funds, code, etc. - that can be applied to this problem. We're looking for people with expertise, vision and leadership capabilities."
MoCo's raison d'être isn't Firefox, it's improving the Internet through Firefox and other initiatives. That's why it's investing in the mobile space with projects like Joey, and it's why Mitchell and others (like me) are interested in mail.
But Thunderbird only has 5-10 million users (including me), while Firefox has 100 million, which means that at least 90% of Firefox users (and many more users of other browsers) are not being helped by Thunderbird.
That's not because MoCo hasn't invested enough into Thunderbird, it's because Thunderbird isn't the right solution for those users. And, based on its development trajectory to date, I don't expect that to change.
But neither do the other extant mail apps serve their users particularly well. As Mitchell says, we need a new vision for mail, and for me it's one that combines the portability of webmail with the usability of a desktop app and provides innovative new features for presenting and participating in conversations.
So let's not only focus on what to do with Thunderbird, let's also figure out what to do about mail and how to do it.