It would help to know exactly how you moved the profile. For example, if the copy was made while Thunderbird was running, there may be incomplete data on the profile.

Hey, no, none of the Thunderbirds were running when copying the profile folder, neither were the source computer. But that makes me think that maybe it has been stopped in a hurry last time it ran, as computer's fan get fried and computer had to stop for critical temperature reached, but kill sig has been sent and several applications had the time to close. So you're right, that might be the point of failure :/

you might also want to verify the profile you copies is being used. What you describe sounds like a new profile, not the old one.

Go to Troubleshooting information in the help menu and select the show profile folder option. Make sure the files in that folder are the ones you copied. The developers have managed to turn moving profile data into a rolling disaster that really is just a Pain.

Yes, that's the correct profile, as all my accounts/emails were there (and there isn't any other).

I could fix the original computer and boot it: Thundebird run my profile just fine with my favorites folders as before. So really I don't understand why those two thundebirds give different results with same files. 🤷 (the second being a newer *minor* version, there really shouldn't be any incompatibility in files formats or so)

Many parts of the Thunderbird setup uses what are absolute paths. So if the profile changes absolute location (like the user name in the path changes, or the machine for UNC or the operating system changes to and from windows where those path separators change), those parts just go missing.

Thunderbird does not store anything meaningful anywhere but the profile folder, although I is possible an add-on might.

Personally I think your issue is in the from move from a Debian 12 (Thundebird 115.7) to a Ubuntu 22.04