And it came to pass when Pharaoh stubbornly refused to let us go, that Yahweh killed all the firstborn in the land of Egypt, both the firstborn of man and the firstborn of animal. That is why I sacrifice to Yahweh every male that opens the womb, but all the firstborn of my sons I redeem.’
“stubbornly refused.” The Hebrew is qashah (#07185 קָשָׁה), see commentary on Exodus 4:21.
“that opens the womb.” This is an idiom for being the firstborn. The firstborn “opens the womb.”