I have an old IMAP email for an email service I closed/canceled. Under earlier Thunderbird versions I was still able to view/browse the emails in that account (I had been synchronizing the emails all along, so when I closed the account it was fully synchronized offline). Now with TB 91.2, I can no longer view the messages, and the program complains I have to go online to read them. Turns out that TB managed to wipe-out all the emails from that closed account (I had changed the "server name" to "0.0.0.0" in a failed attempt to prevent TB from ever checking that account; that's been a long-standing bug for years now). I'm guessing the program figured if it couldn't contact the server, that there were no emails on the account and wiped them out. So I restored the appropriate files for that account from backup and brought TBird up again. The email account view in TBird will see NONE of the messages, or the sub-folders of it, even though I can still see the files in their appropriate subdirectories. I might have moved this old directory to "Local Folders" except that TBird is unable to figure out what to do with them there either (they never show up as usable folders).