מכת בכורות

(29) In the middle of the night יהוה struck down all the [male] first-born in the land of Egypt, from the first-born of Pharaoh who sat on the throne to the first-born of the captive who was in the dungeon, and all the first-born of the cattle.

(30) And Pharaoh arose in the night, with all his courtiers and all the Egyptians—because there was a loud cry in Egypt; for there was no house where there was not someone dead.

עד בכור השבי UNTO THE FIRSTBORN OF THE CAPTIVE — because they rejoiced at the misfortune of the Israelites (Midrash Tanchuma, Bo 7); and a further reason why they were slain is that they should not say, — if they remained alive — it was our god who brought punishment upon our oppressors, the Egyptians (Mekhilta d'Rabbi Yishmael 12:29) The firstborn of the handmaid, threatened with death in 40:5 but not mentioned here as having been slain, is included in those stated here to have been slain since it enumerates here (i. e. the terms used here are intended to include everyone) from the most important amongst all of them to the least important, and the firstborn of the handmaid as belonging to the Egyptian people, is certainly more important than the firstborn of the captive (cf. Rashi on 11:6).