Tested in the Firestorm Citadel:
(Everything on depth 24 with 15 bars of health)
Damage from FoV with 0.0 bars resistance equals about ~2.8 bars.
Damage from FoV with ~2.5 bars resistance equals about ~2.0 bars.
Damage from other sources with 0.0 bars resistance equals about ~2.0 bars.
Damage from other sources with ~2.5 bars resistance equals about ~1.8 bars.
I wouldn't post this as a bug if everything had a different kind of fire, but as far as I can tell that is not the case. (Zombie breath and fire tiles cause the same fire for example.) While I understand fire should be encouraging certain equipment - nearly 3 bars of damage PER TICK and 2.1 bars despite having appropiate equipment is just overkill. Oh, and while we are talking about ticks:
Ticks from FoV with 0.0 bars resistance are 5.
Ticks from FoV with ~2.5 bars resistance are 4.
Ticks from other sources with 0.0 bars resistance are 4.
Ticks from other sources with ~2.5 bars resistance are 3.
Whelp, a FoV fire with 0 resistance will bring you down to 1 bar from 15 bars of health. Just. Like. That.
This has to be a bug - FoV doesn't has a "chance of fire", that's a "chance of hellfire" we got here.
Shields don't contribute to your defense. Why did you bother to try it?