re :Have copied the old profile (all my old mail) What exactly did you copy? For example, old Windows 8.1 computer would have stored all the Thunderbird profile data here: * C:\Users\<Windows user name>\AppData\Roaming\'''Thunderbird'''\Profiles\<'''Profile name'''>\ Default 'Profile name' folder is usually 'xxxxxxxx.default' where the x's are letters and numbers' Did you copy the 'Thunderbird' folder which contains everything including the 'profiles.ini' file? OR Did you copy the 'Profile name' folder ? re :''I am not being allowed to just copy and paste the '''mailboxes''' into the existing Thunderbird profile'' The new profile in a MAC will probably be located here: * ~/Library/Thunderbird/Profiles/<Profile name>/ The tilde character (~) refers to the current user's Home folder, so ~/Library is the /Macintosh HD/Users/<username>/Library folder. When you installed Thundrebird onMAC and run first time it would create a new default profile. . Did you create a mail account or not? What files/folders do you call 'mailboxes' ? Are you refering to mbox files? Post an image showing what it is you are copying to offer clarity. Post image showing where you are copying them to. Is Thunderbird closed - not running - you cannot paste in anything if TB is running ?