Firstborns are important in the bible.  They inherit the wealth of the father.  At times, the firstborn is not an exemplary person.  Take Cain, for example, who killed his brother.  On there other hand, take Isaac, who was the firstborn of Abraham according to the promise given him by God.  He was the firstborn according to promise and God tested Abraham to offer him in sacrifice.  Abraham's trust in God, proved to be the blessing of all nations, for his descendants gave birth to the Christ child, the firstborn of Mary, and the firstborn from the dead for the salvation of the world.