Depending on how badly you'd be hit, it might not be fatal — in any case, it would make more time for those struck to get help from those who would be able to deal with the pulsefire and extinguish it, thus saving them from a formerly-certain death. The injuries it leaves are burns that turn a strange purple colour and feel cold instead of hot, but are incredibly painful like a normal second-to-third degree burn would be.

It could, yes! The pulsefire is a compound of the ore used to create it, and it draws its power from the suns and moons of the planet, so for an explanation, Newt could either create something that interferes with the chemical compound and makes it work differently, or interfere with its access to the light of the suns/moons, as that would limit its efficacy as well.