The firstborn were already sanctified in Egypt (See Shemos 13:2). Why were they only redeemed now?
Ramban: Because until now, bearing in mind that the Kohanim had not yet been appointed, there was nobody to whom to give the redemption. 1
Which firstborn is the Pasuk referring to? When were they born?
Which firstborn animals Bechoros did the animals of the Levi'im except from the Bechorah?
Rashi: They exempted the Pitrei Chamor (the lambs that Yisrael would otherwise have been obligated to give the Kohanim to redeem their firstborn donkeys). 1
Rashi: One lamb of a ben Levi was sufficient to redeem any amount of Pitrei Chamorim of a Yisrael. Proof of this lies in the fact that the Torah does not count the leftover lambs of Yisrael.