Who was David's firstborn son, born to him in Hebron?

Answer: Amnon

David's firstborn in Hebron was Amnon, by Ahinoam of Jezreel; Absalom was the third son and Adonijah the fourth.

“And sons were born to David in Hebron: His firstborn was Amnon, by Ahinoam of Jezreel; his second was Chileab, by Abigail the widow of Nabal of Carmel; his third was Absalom, the son of Maacah daughter of King Talmai of Geshur; his fourth was Adonijah, the son of Haggith; his fifth was Shephatiah, the son of Abital;”— 2 Samuel 3:2-4 (BSB)