Firewalker can be found at apartment 202 in Sharpton Projects. To reach that apartment you have first to find apartment 306 on the 3rd floor hallway (sharpton interior). From there you climb down to the 2nd floor and head west, through a firedoor. The door has only a low quality keyed lock, so it is easy to pick. At the 2nd floor of sharpton interior is apartment 204, which is also secured by an low quality keyed lock, together with an Ghost in that apartment. Finally in apartment 204 is a hole down to apartment 202, but you cant reach that place due to extreme heat which the ghost Moms is producing. Kill that ghost with EMP grenades, climb down and you will get Firewalker.
This mutation grants:
+5-8 burn resistance +1-4 radiation resistance +1-4 acid resistance
The extra bit of burn resistance is always helpful for vampires.
Takes up a soak slot.