After installing Ubuntu 20.04.1
Restart, the system monitor shows: the swapfile is set to absurdly small 2 GB. This guide corrects this to 24 GB.
Just copy the folder ".thunderbird" to your new notebook, start Thunderbird, everything there, everything works. Simply wonderfully simple.
After installing and logging in everything is there, all passwords.
There is either a folder ".program-name" or ".config/program-name". Copy this folder, install the program and you can use the new and the old one.