What happened to every firstborn in Egypt on the night of the last plague?
On the final plague, the death angel passed over Egypt, and every house in Egypt lost its firstborn. The firstborn of people and even livestock died. Israel could escape by faith, applying the Passover lamb's blood to the doorposts and remaining inside, so the death angel would pass over them.
Les explains that God had been pouring out plagues upon the Egyptians, and the final one was the worst of all. It struck home to families and parents through the death of the firstborn. The angel would fly over, and every house in Egypt would lose the firstborn; everything firstborn, even the livestock, would die. But Israel had an out, dependent on their faith: if they put the blood of the Passover lamb on the doorposts and stayed within the safety of their home, the death angel would pass over. This began what the Jews still celebrate today in the Spring as Passover. Les also notes that Passover and Atonement are two totally different Feast Days, with Passover in the Spring and Atonement, or Yom Kippur, in the Fall. Christ is our Passover, and the protection came by the blood of the Lamb.