it is safer, so that you know you didn't forget anything, to save
everything under ".mozilla-thunderbird".
This way you should be able to remember what extensions you had also.
Of course the OP and you can save how you want to but if you just want
emails then you want to save everything under the "Mail" folder within
the "********.default"
folder.
For future reference you could setup a gmail account and using imap save
all your important emails to that account and not have to worry about
where they are or if you accidentally delete the local copy.