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Assuming you left profile in default location on old PC, the only safe way to do a direct copy is to copy from c:\users\<youri>\appdata\roaming\thunderbird to the same location on new PC. This to be done with TB not running on old pc when copy created and TB installed and not running on new PC. That approach ensures integrity of the profile and the profiles.ini file. If you are just copying the profile itself, I suggest you let TB do the work and use tools>import>import from another installation. Yes, if you have already moved the profile to wherever, you can directly edit the profiles.ini file in default location, but that can also be a can of worms if not careful. A safer option there would be to start profile manager and, with TB's help, select 'create profile' and then select the location of where you stored the profile.