-Fire traps: when triggered, they burst into a small pool of burning oil.
-Mirror traps: Archimedes' heat ray. Scorches enemies in a line. (Could work as a siege engine instead of a trap, too.)
is it actually possible to make them?
as for the Fire Dwarves:you could make the "Fire" in their name not stand for actual fire,but for their fiery temper:since you are basing them on Slayers,you could make them prone to rage,give them unique and effective melee weapons, and also throwing axes and such but weaker armor,and turn them into a frail but deadly army of berserkers.you could also let them brew combat drugs,which give them useful stuff like NOFEAR,NOPAIN,NOSTUN and stat increases,but leave them dizzy or worse after some time has passed.
or,if you want them to have fire based powers,you could create a special caste called "magma priests" which have a very low POP_RATIO,but are fire immune and have special fire based combat interactions:destructive in combat,but rare and prone to collateral.you could also add a workshop which allows regular dwarves to do these fire based interactions.
also,if you can use the WEATHER_FALLING_MATERIAL tag to make it rain a particular material(i don't know,i haven't tried it),you can make reaction that cause magma rain for a significant offering like a lot of gold,or your enemies' skulls.