<< Second, “firstborn son” has a specific significance within Judaism. >>
“Firstborn” had significance in all ancient Near East cultures – and in modern western cultures, for that matter. However, the meaning within the ancient Israelite culture, and especially the Hebrew Bible, is quite different. In point of fact, the heritage or lineage or tradition is usually NOT passed through the first-born, in the Hebrew Bible. Abraham’s heritage goes through his second-born son Isaac, not through his first-born Ishmael. Isaac’s first born son Esau is rejected and his second-born son Jacob receives the blessing. Jacob’s successor is Joseph, 11th born. Moses has an older brother and sister. And so on.
David is the youngest son. Solomon is the youngest son. The following kings – most of whom are terrible – tend to transmit through the eldest son.
One of the amazing bits of the Hebrew Bible is that it rejects the notion of inheritance based on birth-order, and stresses virtue instead, almost always rejecting the first born.