from
Easton's 1897 Bible Dictionary
First-born
sons enjoyed certain special privileges (Deut. 21:17; Gen.
25:23, 31, 34; 49:3; 1 Chr. 5:1; Heb. 12:16; Ps. 89:27). (See
{BIRTHRIGHT}.)
The "first-born of the poor" signifies the most miserable of
the poor (Isa. 14:30). The "church of the first-born" signifies
the church of the redeemed.
The destruction of the first-born was the last of the ten
plagues inflicted on the Egyptians (Ex. 11:1-8; 12:29, 30).
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."