All the first-born. The law of primogeniture prevailed in Egypt, as among most of the nations of antiquity. The monarchy (under the New Empire, at any rate) was hereditary, and the eldest son was known as erpa suten sa, or "hereditary Crown Prince."
Estates descended to the eldest son, and in many cases high dignities also. No severer blow could have been sent on the nation, if it were not to be annihilated, than the less in each house of the hope of the family—the parents' stay, the other children's guardian and protector.
Who sitteth. "Sitteth" refers to "Pharaoh," not to "first-born." The meaning is, "from the first-born of the king who occupies the throne to the first-born of the humblest slave or servant. This last is represented by the handmaid who is behind the mill; since grinding at a mill was regarded as one of the severest and most irksome forms of labour.
The work was commonly assigned to captives (Isaiah 47:1, Isaiah 47:2; 16:21). It was done by either one or two persons sitting, and consisted in rotating rapidly the upper millstone upon the lower by means of a handle.
All the first-born of beasts. Not the first-born of cattle only, but of all beasts. The Egyptians had pet animals in most houses, dogs, apes, monkeys, perhaps cats and ichneumons. Most temples had sacred animals, and in most districts of Egypt, some beasts were regarded as sacred, and might not be killed, their death being viewed as a calamity.
The loss of so many animals would consequently be felt by the Egyptians as a sensible aggravation of the infliction. It would wound them both in their domestic and in their religious sensibilities.