I guess this has happened because your email account uses IMAP. The main feature of IMAP is that whatever is (or is not) on the server is also (or not) in 'subscribed' folders in your email client. It is designed this way so that you can manage a single email account from multiple devices. For example you can manage your email on your PC and on your phone and via the browser on your tablet: they should all be 'in sync' with each other. Therefore it doesn't matter which email client you use instead of Thunderbird: they will all behave the same way if you have an IMAP account. There are ways to avoid this problem in future. One is to use a POP account instead. Many email providers let you use IMAP or POP (though not both at the same time). Another approach is to have some folders that are 'subscribed' and are therefore synchronised with the server and others that are not. Thunderbird offers you the Local Folders account for that purpose: any emails that you store in there are not on the server and are therefore unaffected if you delete mails from server folders. I suggest you search on line to learn about IMAP and POP, there are many websites which give good explanations of the differences between them. You need to have a grasp of the basics in order to make the choice that is right for you.

I don't know how it got to IMAAP. I canceled account and re-did it as a POP3 account. Hope that helps. Talked to my server people, (Centurylink) about recovering my lost e-mails from their server, but they say they are gone and knew nothing about Firefox and couldn't help me. They were useless!! Hope switching back to POP3 helps. Thanks