Why by the plague of the firstborn did the Jewish people need to do something to protect themselves? Why by this plague was Pharaoh informed when the plague would arrive? The plague of the firstborn unlike the previous nine was to punish. Many Jews were idol worshipers and they may have needed to be punished as well therefore they had to protect themselves. The plague occurred at midnight because at that moment in time spiritually God's love for the Jewish people is apparent.