If you downgrade Mozilla Thunderbird or switch from a beta or nightly release back to a stable “release” version, Thunderbird forces you to create a fresh profile, with no mails, no settings and no nothing left.

At the moment there is no reason for this and there is a small command that still enables you to force thunderbird to accept the version you’re switching to.

Make sure to backup your old profile before doing this!
Open the thunderbird folder and type this:
thunderbird -P –allow-downgrade

This opens the profile window of thunderbird, where you can select the default profile.

From now on it should work with your normal profile without any error messages.