Answer
$W_{ext} = 0$
$U_{Gf} = 0$
$K_f = 2~J$
Work Step by Step
Since there is no force acting on the firefighter except for the gravitational force and the frictional force which is accounted for in $\Delta E_{th}$, then $W_{ext} = 0$
Since the firefighter is on the ground at the end, $U_{Gf} = 0$
Since energy must be conserved, $K_f = 2~J$