Is this problem on a web-based mail site you use in Firefox, or in folders you access using the Thunderbird email program?

With Firefox, the mail folders are controlled by the server and you're just viewing them with the browser. In case Firefox is somehow misinterpreting what's on the server, could you check using a different browser such as Edge or Internet Explorer.

With Thunderbird, mail can be viewed locally either in "Local Folders", which are largely independent of the server, or in IMAP folders, which mirror what is on the server (with two-way synchronization). A Thunderbird expert can provide more information if this question is about Thunderbird.

You helped me focus my question and add important facts. My problem has occurred using Yahoo mail on Firefox. I do not use the Mozuilla Thunderbird email client. Do you think I should be directing my problem question to Yahoo and not to Mozilla Firefox? Also, I have the same problem of mysteriously deleted email folders when I access my Yahoo email using the Chrome browser.

Hmm, that's a bad sign. I think you may need to check with Yahoo. They also should be able to tell you if your account was accessed in any other manner that could have affected your mail.

I agree that I need to follow up directly with Yahoo. Thanks for helping me figure out a more appropriate way to try to solve my problem. I don't believe this is a problem that Mozilla can help me solve.