1. Mozilla Corp. dropped Thunderbird as one of their projects, and told the community they would have to figure out how to move ahead with the project if they wanted to. News of "Thunderbird is dead" spread out after this announcement.
2. It turned out there was plenty of interest in the project among the public. Donations came in, and the project was kept alive. They established the "Thunderbird Council" as their governance body.
3. The project still needed some legal home and assistance. The Mozilla Foundation (the non-profit) offered to serve as that. The council studied several options and decided to accept this offer.
And this is how they got here: https://www.thunderbird.net/en-US/about/
[1]: https://www.thunderbird.net/en-US/thunderbird/68.0/releaseno...
[2]: https://www.thunderbird.net/en-US/thunderbird/60.0/releaseno...
The layman hates updates.