Our tractate deals with three kinds of firstborn. First is the firstborn of man, redeemed for five sheqels (Num. 18:16). Then there is the firstborn of a clean beast, for example, of the flock or herd, which is deemed holy and is given to the priest, whether or not it is blemished. If it is not blemished, it is offered up on the altar and its flesh is eaten by the priests as Lesser Holy Things (M. Zeb. 5:8). If it is blemished, it belongs to the priest and is slaughtered as an unconsecrated animal and eaten without restriction. The third is the firstborn of an unclean beast, signified by an ass (Ex. 13:13), which is redeemed in exchange for a lamb or the neck of which is to be broken. The lamb belongs to a priest. The ass then is deemed unconsecrated, and the lamb, which belongs to the priest, likewise is unconsecrated. The tractate treats the firstborn of an ass (Chapter One), then the firstborn of clean cattle (Chapters Two through Eight), and finally the firstborn of man (Chapter Nine).
Maimonides (Firstlings 1:1-8, Danby, Offerings, pp. 63-64) provides a brief introduction to the substance of the law:
1. It is a positive commandment to set apart all males which open the womb, whether among men or among clean cattle or among any kind of ass, whether they are unblemished or whether they are terefah, for it is said: Sanctify unto me all the firstborn, whatsoever openeth the womb among the children of Israel, both of man and of beast (Ex. 13:2). And these must all be given to the priests.
2. The firstborn of man and the firstborn of an ass must be redeemed, and their redemption price falls to the priests. The firstborn of clean cattle must be slaughtered in the Temple Court, like other Lesser Hallowed Things, its blood tossed, and its sacrificial portions burned...and the rest of the flesh is to be consumed by the priests, for it is said: Howbeit the firstborn of man shalt thou surely redeem....But the firstling of an ox thou shalt not redeem; they are holy...and the flesh of them shall be thine (Num. 18:15-18).
