The term “firstborn” is a single word and it refers to rank, it does not relate to being born, as the following shows:
“Jesus Christ was resurrected on Nisan 16, 33 C.E., the day when the Jewish high priest offered up the firstfruits of the barley harvest. This fits in accurately with Jesus' being the ‘firstfruits’ in the resurrection of the human dead. (1 Cor. 15:20) This put Jesus Christ in the first ‘rank.’ Just as in the Jewish barley harvest there were afterfruits to be reaped, so too there must be afterfruits in the resurrection: of the dead. But since Jesus Christ ranks first, Paul called him ‘the firstborn from the dead, that he might become the one who is first in all things.’”—Col. 1:18. (The Watchtower, June 15, 1979)