'Firstborn Son' in the Bible

These are the heads of their fathers' households: The sons of Reuben, the firstborn son of Israel, were Hanoch and Pallu, Hezron and Carmi. These were the clans of Reuben.

and all the firstborn in the land of Egypt will die, from the firstborn son of Pharaoh who sits on his throne, to the firstborn son of the slave girl who is at her hand mill, and all the firstborn of the cattle.

When Pharaoh stubbornly refused to release us, the Lord killed all the firstborn in the land of Egypt, from the firstborn of people to the firstborn of animals. That is why I am sacrificing to the Lord the first male offspring of every womb, but all my firstborn sons I redeem.'