John Gill Commentary Psalms 105:36

"He smote also all the first-born in their land, The chief of all their strength." — Psalms 105:36 (ASV)

He smote also all the firstborn in their land
Both of men and beasts; the firstborn of the king on the throne, and of the maidservant behind the mill, and of the captive in the dungeon; this was the last plague, and which prevailed upon the Egyptians to let Israel go, (Exodus 11:5) (12:29-33) .

The chief of all their strength ;
or the first of their strength; the same in different words as before, their firstborn; see (Genesis 49:3) .