You provided no details. If this was a change from one email address to another, you would be required to movl all the messages yourself.

I DID specify servers, as in Rogers/Shaw to Telus which necessitates a change of email address after the @! That does not explain why all my folders were emptied of my delivered and saved emails!

Well, I don't know your specifics, but generally, when a person changes from one email provider to another, the only way existing messages will get to the new server is if the user manually forwards them. Email providers do not usually see it as their responsibility to do that. Your new email provider sets up the account with the new email id and waits for it to be populated by incoming mail to that address. What am I missing here?

Those WERE my specifics! I'm talking about all my folders all being emptied of all saved emails over years all at once! That shouldn't happen regardless of anything!

The most likely scenario is that your account is IMAP, and you did modify the existing account instead of creating a new one for the new server. The new server has no messages yet, hence it did erase all your mail which did previously exist on the old server. This isn't Thunderbird's fault, it's just how IMAP works. You probably should have spent a little more time prior to the switch of the account to determine what needs to be done to preserve your messages.

All that is my best guess, since you provided virtually no details about your setup. Also guessing, you don't have backups of your Thunderbird profile.

You might try contacting the prior provider to see if the account can be reinstated from a backup for a week or so to allow time to move the contents.

It looks like you're talking about some IMAP mail account, you changed from some mail host provider to another not understanding how IMAP works, and are now blaming Thunderbird for something that was entirely your fault…

This is pure guess based on your absolute unwillingness to provide any details, though. An entirely different scenario might also be possible, but I'll leave it at that because I consider it a waste of time trying to help someone that's unwilling to properly describe his problem…