What is the significance of the fact that Pharaoh was the first to rise at midnight?
Rashi: He was the first to rise and go round the houses of his servants to arouse them and inform them of the calamity that had befallen them. 1
See Sifsei Chachamim.
Surely there were houses in which the Bechor had already died! How can it be that there was no house without a Mes?
Rashi: If there was no firstborn in the house, then the oldest member of the family died. 1
Da'as Zekenim #1, Hadar Zekenim #1, from Sifri: Their custom was, when a Bechor died, they made an image of him. Those images melted, and it was as harsh for them as the day of death.
Da'as Zekenim #2, Hadar Zekenim #2: Mice dug up the firstborns from where they were buried and dragged them [to the houses].
Rosh: The Egyptians buried their dead in their houses, and dogs dug up the corpses and dragged them from house to house.