I think I need to start with some conventions. Thunderbird is a program, it runs on your computer, it is not a "site" As a mail client Thunderbird collects mail from mail servers and in the case of IMAP mail accounts maintains a synchronised set of folders with the mail server. You say "I decided to change, on my Mac Pro, the default email client in Thunderbird from Yahoo to Gmail" I assume that you mean you chose to change the mail provider in Thunderbird from Yahoo to Gmail. How you did that I have absolutely no idea, but I am guessing from your discussion of catastrophic results that you edited server names in account settings rather than add another mail account to Thunderbird ie having gmail and yahoo at the same time. So if you took a synchronised folder of mail from yahoo and tried to update the server involved to gmail. The net result would be the gmail server would never have heard of any of the yahoo mail so it would be deleted and then the same would have happened most likely to the contents of the gmail account as well. So before you do anything else, please explain exactly how you did what you did. There is a slim chance some mail may be recovered, but that is dependent on exactly what you did.