Jamieson, Fausset and Brown on Hebrews 11:28

28. kept--Greek, "hath kept," the Passover being, in Paul's day, still observed. His faith here was his belief in the invisible God's promise that the destroying angel should pass over, and not touch the inmates of the blood-sprinkled houses (Ex 12:23). "He acquiesced in the bare word of God where the thing itself was not apparent" [Calvin].

the first-born--Greek neuter; both of man and beast.