Have you considered that the firstborn of all creation is signifying the Creator? Pick a species and say that first born of that species on the day it was created, is where all the different variation of that species came from. So the firstborn of all creation is signifying where all creation came from as from the Creator, would it not?
John did say that no man had seen God at any time, but John did report twice that Jesus said that He has seen God the Father;
But scripture in the O.T. as written by Moses as led by the Holy Spirit, testify of men having seen God; so when you put them all together; no man had seen God the Father, but men had seen God the Son in the O.T. BEFORE His incarnation.
The Lord had appeared to Abraham, Isaac, & Jacob and even Moses.... and He spoke through the prophet Isaiah in Isaiah 48:16-17 where the Lord God is speaking and yet it is the Lord God and His Spirit that will send Him as that prophesy was fulfilled in Matthew 3:15-17 at Jesus's water baptism.
Isaiah 48:16 Come ye near unto me, hear ye this; I have not spoken in secret from the beginning; from the time that it was, there am I: and now the Lord God, and his Spirit, hath sent me. 17 Thus saith the Lord, thy Redeemer, the Holy One of Israel; I am the Lord thy God which teacheth thee to profit, which leadeth thee by the way that thou shouldest go.