Maybe hidden by quick filter?

Don't have any filters that I know of. Deleted and reinstalled everything. Repaired folders. Never came back. Used same THUNDERBIRD directory. Just changed it to .old, reinstalled, and renamed it back. Guess I'll start over from scratch. My emails aren't rocket science.

In which folders were/are those eight-to-ten messages?

Outlook INBOX. Checked both on-line sites; weren't there.

Do you access your mail from other clients? Do you use it as a POP account from anywhere?

I do not. Both Outlook and GMail are both IMAP.

If it happens again, see if Wayne is right: If quick filter options are shown above your message list, click on the "Quick Filter" button to hide them. See if your missing messages appear.

I have read that anti-virus software can quarantine all messages in a folder if it suspects even one because all messages in a folder are in the same mbox file and anti-virus operates at a file level. If you are running anti-virus software, you may want to create an exception for Thunderbird in it and/or check the option in Thunderbird settings > Privacy & Security to enable quarantining of individual messages.