Ephraim was not a firstborn. He had an older brother named Manasseh.
Regardless, neither Manasseh, David, Esau, or any other firstborn Israelite child, was ever called "my firstborn son" by God the Father.
The dispensational confusion, touches the Gospel message, big time.
There is only one Son of God, who became flesh . . lived, died, and resurrected in that flesh, to sit again in His glory with the Father in heaven.
There is only one Jesus Christ. One church body, of which He is head.
One could drop the word "firstborn" as seen multiple times in the bible, and Gospel doctrine would not change, but one cannot drop the "only begotten" from the description of God's Son, without damaging the teachings of the Trinity, Hypostatic-Union of Christ, and the Gospel message that has the power to save.
But dispies think nothing of denying that "Israel" is a name of the only begotten Son of God.