Is these two Windows PCs? If so, assuming you did not change default physical file locations, do this: - exit TB on old PC - copy c:\isers\<yourid>\appdata\roaming\thunderbird to external media - install TB on new PC if not already done. Be sure to NOT be running TB at this time - copy the backed up thunderbird folder from the external media to c:\users\<yourid>\appdata\roaming on new PC, overlaying existing files. - restart TB and all should be there.