Firstly, it should be noted that all of Noah’s family had been blessed by God (v1), and these were the seed from whom the whole earth would be repopulated. Yahweh thereby declared His love and kindly intentions towards postdiluvian humanity, affirming a few verses later that he regarded fallen man as being in His own image (v6). Noah had three sons, one of whose seed became cursed, Canaan, son of Ham who had uncovered his father’s nakedness. Japheth, prefiguring Ishmael, retained his father’s blessing but was not chosen to be the seed from which the children of Israel would stem. That was to be Shem via his firstborn son Arpachshad. This is one of several biblical indicators of three rather than two soteriological states of humanity, a theme worked out in detail in my book* demonstrating the munificence of God’s providential care.
There is a further motif evident here pertaining to the firstborn being the line of special blessing. This can be traced back to Adam’s son Seth; firstborn heir to Adam by default, his two elder brothers Cain and Abel having been respectively banished and murdered. Seth’s firstborn was Enosh, the first to evoke the Lord (Gen4:26). The re-occurring phrase within these genealogies is in the format: “When X was a certain age he fathered Y. X lived for so many years and went on to father (other) sons and daughters”, only the firstborn son ever being named (Gen5). After Enosh came Kenan, then Mahalalel, Jared, Enoch who “walked with God”, Methuselah who outlived his son Lamech, father of Noah, dying within a year of the Flood. There is no reason not to take the patriarch’s ages literally, especially in view of Gen6:3.
A similar motif continues with the Church and her Head. The “Firstborn of all creation” is Jesus Christ (Col1:15); the firstborn of God’s children are the elect, who are also described as “the church of the firstborn” (Heb12:23 Greek: “firstborn [prototokon] is plural – it is not referring to Christ Himself but His chosen people). Under the Old Covenant with Israel, the firstborn son was always “consecrated unto the Lord” (Ex13:2). Those designated “firstborn” are typically designated to sanctity and kingship, and by them are the whole family to be blessed.
* A free PDF of e-book available HERE