https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer

That usually indicates the profile was not copied correctily. if the old PC is still available (and you're using Windows) try this: TRANSFER::: Here is the link for moving a profile in Windows when installed in default location: https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer here is the info in brief steps: # ensure thunderbird is not running on old machine # enter %appdata% in search window on task bar and press Enter key # this should open windows file explorer in Appdata\roaming folder # highlight the Thunderbird folder and copy to external media # ensure thunderbird is installed on second computer and NOT running # enter %appdata% in search window on task bar and press Enter key # you should be in Appdata\roaming folder , copy&paste the exported Thunderbird folder there. This may prompt to overwrite an existing folder of same name. allow it # start thunderbird and all should be there