thunderbird-91 with Unix Mail broken

Up until the change to thunderbird-91 I could use the builtin Unix movemail to get mail from my local postfix server folders (/var/mail/<me>) into thunderbird and send outgoing mail thru postfix. Thunderbird-91 does not recognize my existing .thunderbird/xxx.default/Mail/localhost folder, although it is still there. I can rename the folder in "Local Folders" and see old mail, but cannot get new mail. I had a Slackpkg from March 2021 for thunderbird-78 and reverted to that, and a week old .thunderbird folder backup and reverted to that, since -91 changed .thunderbird in some subtle way. So I have a "workaround" but no fix....