In this instance you should really do the Firefox backup data method so when you log out of the old computer you can do a data transfer of Firefox data and profile to insure the new computer has all the data inhouse. And as you just last mentioned since it was sold and gone and formatted all the data was there is gone and sync didn't get all the data synced. Remember sync isn't a data backup only syncing devices. And since the old computer is gone any options to copy and move data is impossible now and that data is now gone. As now there is nothing much anyone else can really help as there is no real possible way to get the data now.