I am not sure if you aware but Thunderbird was indeed droped by mozilla and now it is independent project, with quite a lot money from donations so they seem to be good for the future
That's right, it had been so long that I had forgotten. Thanks for reminding me.
Edit: Taking a further look into the history, it looks like the project has been under a lot of turmoil WRT shutting down and being revived. Explains why I have avoided it for so long.