Easton's Bible Dictionary
The "first-born of the poor" signifies the most miserable of the poor (Isaiah 14:30). The "church of the first-born" signifies the church of the redeemed.
Menephtah is probably the Pharaoh whose first-born was slain. His son did not succeed or survive his father, but died early. The son's tomb has been found at Thebes unfinished, showing it was needed earlier than was expected. Some of the records on the tomb are as follows: "The son whom Menephtah loves; who draws towards him his father's heart, the singer, the prince of archers, who governed Egypt on behalf of his father. Dead."
First-born, Redemption of
From the beginning the office of the priesthood in each family belonged to the eldest son. But when the extensive plan of sacrificial worship was introduced, requiring a company of men to be exclusively devoted to this ministry, the primitive office of the first-born was superseded by that of the Levites (Numbers 3:11-13), and it was ordained that the first-born of man and of unclean animals should henceforth be redeemed (18:15).
But the first-born of unclean animals was either to be redeemed or sold and the price given to the priest (Leviticus 27:11-13, 27). The first-born of an ass, if not redeemed, was to be put to death (Exodus 13:13; 34:20).
First-born, Sanctification of the
A peculiar sanctity was attached to the first-born both of man and of cattle. God claimed that the first-born males of man and of animals should be consecrated to him, the one as a priest (Exodus 19:22, 24), representing the family to which he belonged, and the other to be offered up in sacrifice (Genesis 4:4).