Try this: export the address books to LDIF format, copy the existing profile folder to a safe location, with TB closed, uninstall TB 90, install TB 78, but don't run it at the end of the installation. Open a Run window with Winkey+R, type thunderbird.exe -p --allow-downgrade and select the profile that was run in 90. If the address books don't appear, import the LDIF files.

Thank you for your reply but it won't work on the computer in question. There are 4 users of the computer that have three different Windows logins, only one of whom is fully computer literate. Among the 4 users there are 10 e-mail accounts with several service providers, some of whom have to be phoned every time a password is required because one or more users used an old password or didn't write down the new one. One user has more than 10 000 messages in his inbox and would be devastated to loose them. I'm afraid that if I made even one mistake I would throw an already chaotic family computer into more chaos.

Could a "downgrade" button or a single line of code not be provided that would automate the process for all users simultaneously?

There's no one-click method to achieve what you want, but you can wait and see if someone else provides a simpler way. You only have to install 78 once in an administrator account, but the command line must be run for each user account, as the profile names will be different.

I suggest you wait until version 91 comes out. When it does, there will be less risk in converting to version 91 from beta 92 or beta 91.