I created an IMAP account with Thunderbird and downloaded all messages.

This only makes sense if you also did turn on synchronization for the IMAP account. In brief, synchronization means that Thunderbird downloads an offline copy of your messages to the local disk.

If you didn't do this, then there's probably little what has actually been downloaded, may be a few message bodies for messages you opened. What you did download were just the message headers.

Now that the old IMAP account isn't working anymore, it's too late. The safe way would have been to copy all the messages from the server to the Local Folders account while the old account was still working.

You're not the first one having this problem. What really surprises me is that folks don't ask questions about this prior to migrating to a new provider, but only once they realize something went wrong.

This only makes sense if you also did turn on synchronization for the IMAP account. In brief, synchronization means that Thunderbird downloads an offline copy of your messages to the local disk.

Correct. That's what I did, of course. I turned on synchronisation and told thunderbird to download all messages completely, not only the headers.

The data is there, by the way. I can open the files with `less` and see all the messages there. That's just not really suitable to browse through them.

Thunderbird does not display anything, it is just empty.

Try to put Thunderbird in offline mode. https://support.mozilla.org/en-US/questions/1022683

Can I just create a local folder and import the files?

Yes, that should work regardless. You may also try this add-on. https://addons.thunderbird.net/thunderbird/addon/importexporttools-ng/

I am using Thunderbird 106, so I can't use the plugin. I tried to build it for myself, but I did not found out how.

I found out that in my real profile directory there was no data anymore -- I had it in another `~/.thunderbird` directory, which I copied to `~/snap/thunderbird/common/.thunderbird`. That is probably the reason why the offline mode did not work before.

No, I just created a new folder in my local folders, closed Thunderbird and replaced the files completely. Beside a few errors (some mails detected incorrectly) this worked and I can live with this as a back up now.

Thanks for the help!

Can I just create a local folder and import the files?

Yes, that should work regardless.

How would this work? Is there another way?

How would this work? Is there another way?

As per your description above it sounds like you already did do this.