Exodus 12:30

Pharaoh rose. From his bed....

At night. And not when it is usual for kings, at the third hour of daylight...

If there was a firstborn -- he died. If there wasn't, the oldest in the house was called a firstborn, as it says (Psalms 89:28) "I also will appoint him first-born". Another thing: Egyptian women would prostitute themselves beneath their husbands and bore children of open men, and would have many firstborns, even five to one mother, each from a different father.

And Pharoh rose up in that night, and all the rest of his servants, and all the rest of the Mizraee; and there was a great cry, because there was no house of the Mizraee where the firstborn was not dead.

FOR THERE WAS NOT A HOUSE WHERE THERE WAS NOT ONE DEAD. Rashi comments: “If there was a firstborn there, he died; if there was no firstborn there, the chief person in the house died because he is called b’chor (firstborn), as it is said, I also will appoint him firstborn. Another interpretation: The Egyptian women led dissolute lives under their husbands and bore children from unmarried young men. Thus they had many firstborn sons, sometimes [as many as] five to one woman, each one being the firstborn to his own father.”
In line with the plain meaning of Scripture, the firstborn that died in Egypt were the firstborn of their mothers, and this is why He sanctified in their place all the firstborn, whatsoever openeth the womb among the children of Israel, both of man and of beast. The firstborn of Pharaoh that sat on his throne was the firstborn to his mother. And such indeed is the custom among kings that the reigning mistress be a virgin, something like it is said in the case of Ahasuerus. But in accordance with the opinion of our Rabbis [who say that the firstborn of a father also died], we shall explain that in Egypt, He smote all their firstborn. That is to say, the firstborn of the father, since he is the first-fruit of his strength, and the firstborn of the mother, since he opened the womb, and also the chief person in the house. Yet it was His desire to sanctify in Israel in their place only the firstborn of the mother, because that is a matter more known and of open knowledge. In cattle, only the firstborn of the mother is known at all, and therefore He chose from among all of them only [the firstborn of the mother]. A sort of proof for this [statement of the Rabbis that the firstborn of the father was also smitten] is the verse: And He smote all the firstborn in Egypt, the first-fruits of their strength in the tents of Ham, for such an expression, [i.e., the first-fruits of his strength], is said only of a male.

כי אין בית, “for there was not a single house, etc.;” the question is raised how it was possible that there were no houses which did not contain a firstborn? (Mechilta section Pessach chapter 13) The fact is that the Egyptians, whenever a firstborn died, displayed a likeness of the deceased in the house he used to live in. At the time when this occurred, each such likeness of a firstborn even if he had died earlier, fell down and broke into pieces, and the inhabitants of the house mourned him as if he had died now. It has also been reported that the place where firstborn had died became infested with mice which dug up the earth where he had been interred. All this caused the survivors to feel the loss of that person as if it had occurred just then.