Job the firstborn?
Here’s a thought. What distinguishes the sons of God? Faith. What distinguishes the firstborn of God? Faithful perseverance through suffering. Compare Job 1:3 to Job 42:12. Job’s inheritance from the Lord has doubled, hinting at the double-portion received by the firstborn son. What took place between these two accountings of Job’s inheritance? A whole lot of suffering.
I can think of at least one other firstborn son who demonstrated his standing via suffering (Philippians 2:5-11).