'Firstborn Son' in the Bible

But his father Isaac said to him, “Who are you?”He answered, “I am Esau your firstborn son.”

These were chiefs of the sons of Esau: the sons of Eliphaz the firstborn son of Esau; chief Teman, chief Omar, chief Zepho, chief Kenaz,

Joseph named his firstborn son Manasseh because, he said, "God has made me forget all of my hard life and my father's house."