He smote
nakah  (naw-kaw')
to strike (lightly or severely, literally or figuratively)
also all the firstborn
bkowr  (bek-ore')
firstborn; hence, chief -- eldest (son), firstborn(-ling).
in their land
'erets  (eh'-rets)
the earth (at large, or partitively a land) -- common, country, earth, field, ground, land, natins, way, + wilderness, world.
the chief
re'shiyth  (ray-sheeth')
the first, in place, time, order or rank (specifically, a firstfruit) -- beginning, chief(-est), first(-fruits, part, time), principal thing.
of all their strength
'own  (one)
ability, power, (figuratively) wealth -- force, goods, might, strength, substance.