'Firstborn Son' in the Bible

“If a man has two wives, the one loved and the other unloved, and both the loved and the unloved have borne him sons, if the firstborn son belongs to the unloved,

and his firstborn son was Abdon, then Zur, Kish, Baal, Nadab,

and his firstborn son was Abdon, then Zur, Kish, Baal, Ner, Nadab,

And she gave birth to her firstborn son; and she wrapped Him in cloths, and laid Him in a manger, because there was no room for them in the inn.