Textual words from them:
It’s our first step towards a more modern, more beautiful, and more customizable Thunderbird experience. We think you’re going to love it, and we are endlessly grateful for all of your support throughout the years 💙
The new website is also pretty epic: https://www.thunderbird.net/en-US/ (bit messed up on mobile)
Unfortunately the flathub version doesn’t seem to be updated.
It works on Chromium browsers. It does not work on Firefox.
Interesting to see even Mozilla apparently testing their websites only on Chromium.