In the time of the ancient patriarchs, the firstborn son received the birthright (Gen. 43:33) and thus inherited the leadership of the family upon the death of the father. The firstborn had to be worthy to assume this responsibility (1 Chr. 5:1–2) and could lose his birthright by unrighteousness.
Under the law of Moses, the firstborn son was regarded as belonging to God. The firstborn received a double portion of his father’s possessions (Deut. 21:17). After his father’s death, he was responsible for the care of his mother and sisters.
The male firstborn of animals also belonged to God. Clean animals were used for sacrifices, while unclean animals might be redeemed or sold or put to death (Ex. 13:2, 11–13; 34:19–20; Lev. 27:11–13, 26–27).
Jesus was the firstborn of the spirit children of our Heavenly Father, the Only Begotten of the Father in the flesh, and the first to rise from the dead in the Resurrection (Col. 1:13–18). Faithful Saints become members of the Church of the Firstborn in eternity (D&C 93:21–22).