While I agree that Thunderbird as a standalone Windows/MacOS/Linux desktop email client is a very important project, and needs to be continued, maintained and refined, the paradigm of email has moved on somewhat since 2003.
People need to be able to do more than just access email on a desktop or laptop.
I'd like to see two new things under the Thunderbird/Mozilla name:
a) a fully open source GPL licensed, self-hostable, webmail server application that fulfills the same functions as rainloop or roundcube, implemented in the Thunderbird name. Maybe it could have its own new GUI to run inside the browser, or it could offer an optional "traditional" GUI that is similar to the Thunderbird desktop client. This has a possibility for a natural symbiotic relationship with Firefox, as the best and most optimal web browser client to view the webmail.
b) a Thunderbird Android email client. There should be no reason why people should be locked into the default Android google/gmail email client. I can't even view message headers on it. I want a full featured "power user" IMAP-over-TLS + SMTP email client for Android that is also open source.