The simplest way to move your Thunderbird data from the old to the new PC would have been to transfer (copy & paste) the entire folder Thunderbird from ... \users\username\AppData\Roaming\'''''Thunderbird'''''\ on the old PC to the same location on the new PC as it is described in this help article https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer In your Thunderbird installation on the old PC your accounts are set up as POP accounts, but in the Thunderbird installation on the new PC you used the IMAP protocol for your accounts. That means that you can download only those incoming messages still remaining on the server and that you would probably need a tool to import all mail files from the old Thunderbird profile to Local Folders in the Thunderbird profile on the new PC. If Thunderbird is still running on your old PC I advise to move your actual "Thunderbird" folder on the new PC to another place such as the Desktop and replace it with the "Thunderbird" folder from the old PC. The application Thunderbird must be closed when you execute this procedure. If you want to keep your actual Thunderbird installation but have access to your old received, sent, archived.... etc messages, addresses, calendar events.... you would have to import all your mail files as well as address book and calendar files separately from the old to the new PC. This method would be more difficult and more time consuming...