Fixes issues:

#2632 Fire Issues (Fire doors work, Flamethrower reverted to making a flamethrower liquid fuel subtype, flame grenades now make a puddle of welder fuel that is lit aflame with napalm)
#2630 (The code has been calling (turf).temperature_expose when it was supposed to be hotspot_expose), #1865 (aqain)(Added a check to remove trace gas fuel under a certain point).
Also: added a flashpoint value for plasma, which is the temperature at which it self-ignites (Currently 246C or 519K which is equal to gasoline)
Partial-tile firedoors now default to full-tile firedoors as the part-tile ones were breaking ZAS zones entirely.  Possible cause of #2627 and maybe #2451, but those need tested and I did not have time.

code/modules/reagents/Chemistry-Reagents.dm

@@ -1121,26 +1121,15 @@ datum
 			reagent_state = LIQUID
 			color = "#660000" // rgb: 102, 0, 0
 
-//Commenting this out as it's horribly broken. It's a neat effect though, so it might be worth making a new reagent (that is less common) with similar effects.	-Pete
-/*
+
 			reaction_obj(var/obj/O, var/volume)
-				src = null
 				var/turf/the_turf = get_turf(O)
 				if(!the_turf)
 					return //No sense trying to start a fire if you don't have a turf to set on fire. --NEO
-				var/datum/gas_mixture/napalm = new
-				var/datum/gas/volatile_fuel/fuel = new
-				fuel.moles = 15
-				napalm.trace_gases += fuel
-				the_turf.assume_air(napalm)
+				new /obj/effect/decal/cleanable/liquid_fuel(the_turf, volume)
 			reaction_turf(var/turf/T, var/volume)
-				src = null
-				var/datum/gas_mixture/napalm = new
-				var/datum/gas/volatile_fuel/fuel = new
-				fuel.moles = 15
-				napalm.trace_gases += fuel
-				T.assume_air(napalm)
-				return*/
+				new /obj/effect/decal/cleanable/liquid_fuel(T, volume)
+				return
 			on_mob_life(var/mob/living/M as mob)
 				if(!M) M = holder.my_atom
 				M.adjustToxLoss(1)