Sforno on Exodus 13:15/ספורנו על שמות י״ג:ט״ו

1ויהי כי בקשה פרעה, seeing that Pharaoh had put so many obstacles in the path of our eventual liberation he is himself compared to a donkey as we know from Ezekiel 23,20, the prophet describing the flesh of the Egyptians as similar to donkeys. Pharaoh could have easily redeemed himself by releasing the Israelites, i.e. the Israelites are compared to the שה, the lamb. As a result of failing to do so, G’d chose to kill the firstborn instead.

3כל בכור בארץ מצרים, the fact that the Torah does not speak about “every Egyptian firstborn,” but about “every firstborn in Egypt,” shows that basically, the Israelites, i.e. their firstborn were also guilty of death at the hands of G’d. He saved them by sanctifying them as His personal property.

4על כן אני זובח, this act of grace by G’d at the time is the reason that I annually express my gratitude by slaughtering the firstborn male of any pure animal. [after giving it to the priest. Ed.] Seeing that the firstborn donkeys or any other impure animal are unfit as offerings on the altar, I redeem them in order to be able to make secular use of these animals as beasts of burden, etc. As far as the firstborn of my own sons are concerned, seeing that they were sanctified from birth, I redeem them in order that they can lead regular lives, as the priesthood is no longer an option for such firstborn ever since the sin of the golden calf.