Hebrews calls Jesus a priest forever in the order of whom?

Answer: Melchizedek

Quoting Psalm 110, Hebrews says, "You are a priest forever in the order of Melchizedek" — not in the order of Aaron (7:11).

“And in another passage God says: “You are a priest forever in the order of Melchizedek.””— Hebrews 5:6 (BSB)