I am using the unofficial AppImage with some semi-inactive email accounts on my testing machine. I took it for a test drive on AlmaLinux, LM-20, and Manjaro. I have not thoroughly tested it, but it appears to be stable on AlmaLinux and LM-20. Thunderbird is installed and fully functional on my Manjaro system - I halted the test when I first launched the AppImage it complained Tbird was already running, which suggests to me it might be accessing the same config file(s).
I do not hate the new look/feel, but I am not thrilled with it, either. It looks more like webmail than a traditional email client. The new Address Book is a very nice improvement.
The verdict: I would discourage anyone from using this AppImage on anything other than a test platform, mainly because it is an unofficial release.
https://github.com/srevinsaju/Thunderbird-AppImage