Firstborn

The Greek term is used 9 times in the New Testament.

Twice, Jesus being His mother's first child, Matthew 1:25 and Luke 2:7.

5 times to refer to Jesus Christ, being part of His creation, being the first to be resurrected from the dead, Romans 8:29, Colossians 1:15, Colossians 1:18, Hebrews 1:5-6 and Revelation 1:5.

Twice in the plural.

Once referring to His church, the firstborn ones, Hebrews 12:23.

And once referring to the firstborn ones God spared in the original Passover, Hebrews 11:28.

 

The Greek is in the plural.
πανηγυρει και εκκλησια πρωτοτοκων εν ουρανοις απογεγραμμενων και κριτη θεω παντων και πνευμασι δικαιων τετελειωμενων

 

Not "of the firstborn ones' "?