There seem to be a lot of people commenting about Thunderbird coming to an end. That's not happening. Today, Thunderbird has two full-time developers working on it. That's been the case for a while now. Thunderbird will still have two full-time developers working on it tomorrow, the day after, and into the foreseeable future.
The changes that are being discussed are not about killing Thunderbird -- actually the exact opposite. The purpose of this public discussion is to figure out how to improve on what Thunderbird has already accomplished by giving the Thunderbird developers a different organizational structure.
Additionally, if you're concerned about the future of Thunderbird, if you use it and depend on it, if you want to see it improving at a faster pace, now is the time to step up and do something.
Are you reporting bugs? Are you testing nightly builds? Are you writing patches? Are you participating in Thunderbird test days? Are you telling your friends and family about it? Are you installing it on machines in your jurisdiction? Are you blogging about it and posting reviews at software download sites? Are you part of the Thunderbird community? If not, and if you care about Thunderbird's future, then step up and get involved.
And for those worried about Thunderbird not being able to survive without the full resources of Mozilla behind it, I'd urge you to look back to those days when a little project called Phoenix, with no organizational support outside of CVS hosting, a Bugzilla product, and a couple of web pages, was able to race past the legacy Mozilla Suite, which had had 5 years of the full resources of Netscape Communications and a large volunteer community behind it, in less than a year. Thunderbird, if it finds the right organization and the right level of community support, can definitely continue to grow and I wouldn't be surprised to see it becoming significantly more than it is today.