daf bit: Bechorot 2-3

We begin a new tractate this week. "Bechorot" is about the first-born. The torah declares that first-born animals belong to the priests and first-born males would except that they were replaced by the Levites. (The father of a first-born son must redeem the son through a ritual called pidyon ha-ben. That is not the subject of today's daf, but I mention it for context.)

The mishna teaches: an Israelite who jointly owns an animal with a gentile, or who sells one to him or receives one from him to be looked after, is exempt from the law of the first-born animal, for it says: "hallowed unto me are all the first-born of Israel" -- Israel, but not of gentiles. The mishna further teaches: priests and Levites are exempt from this law because they exempted the Israelites in the wilderness. (2a, 3b)

The second seems in conflict with the first ("all" doesn't mean "all"?), but is followed by two more pages of g'mara discussing it that I have not digested yet.

Grammar question: I thought a "bechor" was the first-born (son or animal), so I am surprised by the feminine plural ending. Does this construction refer to the mother instead (and I just don't follow the grammar)? If it refers to the young, why not the masculine ending as in "bikkurim", first fruits?