For Thunderbird to properly succeed in providing real choice and freedom, it needs to become the successor to Firefox, in the way that Flock is currently trying to be. (You can think of Firefox as “The Internet 1.0”; Flock as “The Internet 1.1” and Thunderbird as “The Internet 2.0” if you like.)
Each of these is closed and isolated from each other, particularly its direct competitors. Any interconnection is done at the whim of one of these companies, one service provider at a time. I can phone a BT line in Glasgow from a Virgin Media line in York; I can send an email from RandomMail to Mom's Friendly Email Service, even if neither has heard of the other. But if I write a blog post at Acmeblog, my friends using Myface won't see it.
For there to be true freedom, there needs to be an open, standards-based, social network through which people can freely conduct communication. (This is what the Internet is supposed to be in the first place.) And I don't mean “social network” in the limited sense of “a website where you log in and can talk to your friends lol”.
I mean a set of standard protocols by which anyone can communicate with anyone in any conceivable way. I'm thinking of open, federated standards such as email, Atom (including the publishing bit), Jabber, OpenID and OpenSearch (and avoiding saying “semantic web” even though that's pretty close to what I'm on about).
Flock takes a standard web browser and surrounds that with structured social network stuff. Thunderbird should invert that. It should start from a set of high-level concepts such as contacts, presence, subscriptions and messages. Then it should bring in bits of web browseriness as appropriate to display the content.
(By the way, I shouldn't have written this on Asa's website; I should have written it in Thunderbird, to: the web; cc: Asa.)
Imagine Thunderbird and Lightning, Pidgin, Skype (but Free), Miro, AllPeers and the Chandler project, all combined into the only communication program you'll ever need. Thunderbird should be that.