Concise Bible Dictionary:

Jacob when dying said of Reuben “Thou art my firstborn, my might, and the beginning of my strength, the excellency of dignity, and the excellency of power.” This is what he was as the firstborn, for in himself he was “unstable as water” and he should not excel (Gen. 49:3-43Reuben, thou art my firstborn, my might, and the beginning of my strength, the excellency of dignity, and the excellency of power: 4Unstable as water, thou shalt not excel; because thou wentest up to thy father's bed; then defiledst thou it: he went up to my couch. (Genesis 49:3‑4)). He forfeited his birth-right for defiling his father’s bed, and it was given to Joseph, who in Ephraim and Manasseh had a double portion among the tribes (1 Chron. 5:11Now the sons of Reuben the firstborn of Israel, (for he was the firstborn; but, forasmuch as he defiled his father's bed, his birthright was given unto the sons of Joseph the son of Israel: and the genealogy is not to be reckoned after the birthright. (1 Chronicles 5:1)). The law declared that if a man’s first-born son was by a wife he hated, he must not put the son of another wife in his place: the first-born must have a double portion of all that the man possessed “for he is the beginning of his strength: the right of the first-born is his” (Deut. 21:16-1716Then it shall be, when he maketh his sons to inherit that which he hath, that he may not make the son of the beloved firstborn before the son of the hated, which is indeed the firstborn: 17But he shall acknowledge the son of the hated for the firstborn, by giving him a double portion of all that he hath: for he is the beginning of his strength; the right of the firstborn is his. (Deuteronomy 21:16‑17)). Esau is called a profane person for selling his birth-right: it was a privilege God had given him, and which he should have valued as such (Gen. 25:31-3431And Jacob said, Sell me this day thy birthright. 32And Esau said, Behold, I am at the point to die: and what profit shall this birthright do to me? 33And Jacob said, Swear to me this day; and he sware unto him: and he sold his birthright unto Jacob. 34Then Jacob gave Esau bread and pottage of lentiles; and he did eat and drink, and rose up, and went his way: thus Esau despised his birthright. (Genesis 25:31‑34); Heb. 12:1616Lest there be any fornicator, or profane person, as Esau, who for one morsel of meat sold his birthright. (Hebrews 12:16)).

From Manners and Customs of the Bible:

Great respect was paid by the household to the first-born son. He had headship over his brothers; he succeeded to the father’s official authority; He had a special claim to the father’s benediction; in him was the progenitorship of the Messiah; the domestic priesthood belonged to him, according to some authorities, though this is denied by others. Under the Mosaic law he received a double portion of the father’s goods. This birthright could be transferred to another for a consideration, or withheld by the father for cause.