If these are imap folders, with Thunderbird shut down you can delete the underlying files in the Windows file explorer

Well, I sort of solved the problem:

I downloaded a program called 'IMAPsize' and used it to export the contents of the troublesome mbox file to a series of individual *.eml files.

Next, I used the 'search and replace in files' function in Notepad++ to rewrite the 'X-Mozilla-Status:' line to have a code '0001'. For some reason all the messages were reset to some version of 'this message has been deleted."

I then imported all the individual messages using the ImportExportTools add-in.

After all this, I still had a dozen or so messages (out of nearly 800) that were corrupted/damaged.

So that problem was solved, but the question remains is why did this happen to a folder that was not in active use? I didn't add or delete a message in this folder for months if not years.

Why does Tbird suddenly corrupt messages within a folder?

Is there a way to prevent this?

Is the current maildir implementation safer?