Used the positivist firebrand tool and it hostiled an npc. It didn't do any damage.
Regardless of modifications firebrand is a hostile spell. This is no bug.
I did touch the spell a bit, but you'll have to be more clear with this report. Who was the NPC? Racial type? Did any of the bolts actually hit him? Did he just saw you use the spell and he was hostiled? Did it hit him and attempted to heal?
The NPC was the kuo toan captain on the fish isle who gives out the quest. He got hit by a bolt and was hostiled. I wasn't sure if the spell tried to heal him, but it did no damage to him.