At midnight. As prophesied by Moses (Exodus 11:4). The day had not been fixed, and this uncertainty must have added to the horror of the situation. The first-born of Pharaoh. We have no proof that the eldest son of Menephthah died before his father, unless we take this passage as proving it.
He left a son, called Seti-Menephthah, or Seti II, who either succeeded him, or reigned after a short interval, during which the throne was held by Ammonmes, a usurper. The first-born of the captive who was in the dungeon.
This phrase takes the place of another expression, viz. "the first-born of the maid-servant that is behind the mill" (Exodus 11:5). In both cases, the general meaning is, "all, from the highest to the lowest."
This is perhaps the whole that is in the writer's thought; but it is also true that captives in dungeons were in some cases employed in turning hand-mills ( 16:21). And all the first-born of cattle. Rather, "of beasts."
There is no limitation of the plague to domesticated animals.