Of course they did.
The two lead engineers have been working
paid full-time on this for years, where do you think those resources are coming from? The mozilla build team has pushed releases, Thunderbird has had
its place on mozilla.com along side Firefox etc.
Basically, the issue now is - how is Thunderbird going to survive without all this support? I'm not saying it all is going away, but Thunderbird WILL have to do more on its own. As I understand it, Mozilla Foundation/Corporation (not sure which) might still help out somewhat financially, but I am not sure of the status on that situation.
However, I think there's no reason to believe "it's all over". There are plenty of other organizations/companies doing top-notch Mozilla work without being a physical part of Mozilla Corporation. See Joost, Songbird, and
others. Also, it will be interesting to see what high-level decisions are taken by the Thunderbird team now that they will be more independent. I think there's a chance they will have to think more radically about Thunderbird's place in the world than what has been the case up until now.