It's been a weird story. Here's how I remember things going:

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/

They released a new version on the 27th of August[1]. It looks like it that was the first release since August 2018[2].

[1]: https://www.thunderbird.net/en-US/thunderbird/68.0/releaseno...

[2]: https://www.thunderbird.net/en-US/thunderbird/60.0/releaseno...

I believe Thunderbird is now ESR. So it gets a major version then security updates for a year until the next one.

What is the meaning of ESR? Because that sounds like life support to the layman.

Nah. I means Extended Support Release. After Firefox moved to the quicker release train, larger organizations were annoyed because they like stability in order to test stuff prior to release and make sure extensions and everything remains the same. So like Ubuntu (iirc) and some others, they created a Firefox ESR that gets security updates and only major updates ~once a year. I think Thunderbird actually just tracks with Firefox ESR.

Do you want your mail reader constantly tracking the latest web tech?

The layman hates updates.