You purchased a new computer. Is the old one still sitting there running and downloading mail using the POP protocol? That would account for 100% of your started experience as when you set the new one up it would have defaulted to IMAP and be in sync with the server.

Found on https://support.mozilla.org/en-US/questions/1403062

Hi, Matt, thanks for replying so quickly. All contents of my old computer have been transferred to the new a couple of weeks ago and the old one emptied for resale. As mentioned, all seemed to be ok with the new computer for the first several days. I have used Thunderbird for a number of years on the old computer and never had the slightest problem. I've hired a technician, a fine one, but he cannot understand the problem either.

I would still be looking for another device interacting with the email account. Phone, tablet or another computer.

The other alternative is Thunderbird can not actually connect to the mail server (thing like antivirus and firewall are common issues and more recently VPN's as many don't support email and many ISP providers do not support VPNs connecting from outside the ISP provided network.) When this occurs and the account type is IMAP then the contents of folders will synchronize to oblivion, even folders other than the default inbox trash etc will also go missing. Again it looks as you describe because the same mechanism is involved.