Albert Barnes Commentary Exodus 11:5

"and all the first-born in the land of Egypt shall die, from the first-born of Pharaoh that sitteth upon his throne, even unto the first-born of the maid-servant that is behind the mill; and all the first-born of cattle." — Exodus 11:5 (ASV)

Two points are to be noticed:

1. The extent of the visitation: the whole land suffers in its firstborn, not merely for the guilt of the sovereign, but for the actual participation of the people in the crime of infanticide (Exodus 1:22).
2. The limitation: Pharaoh’s command had been to slay all the male children of the Israelites, but only one child in each Egyptian family was to die. If Tothmosis II was the Pharaoh, the visitation fell with special severity on his family. He left no son, but was succeeded by his widow.

The mill—This consisted of two circular stones, one fixed in the ground, the other turned by a handle. The work of grinding was extremely laborious, and performed by women of the lowest rank.

Firstborn of beasts—This visitation holds special significance concerning the worship of beasts, which was universal in Egypt; each district having its own sacred animal, adored as a manifestation or representative of the local tutelary deity.