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."