|Becomes after death|
Fire men come out of the same places as fire imps, but they are a little bolder. Like them, they can go out past the magma pool/pipe and breathe balls of fire that can kill, maim, and/or set targets or material on fire.
After you have made a moat, the firemen should stop attacking directly, but they will still be able to breathe fireballs at your dwarves. You could also try building a wall to go with this moat.
To stop them from attacking your smiths is a little harder, since fire men can swim through fully submerged fortifications and will destroy any wall grates or vertical bars used to try to filter them out. If you only have one tile dug down for the magma below each of your workshops and ensure that said tile is beneath one of the inaccessible tiles of the magma forge, smelter or glass furnace, creatures should be prevented from coming up through the hole.
 Fighting them
Put short, don't, not unless absolutely necessary. They don't feel pain and thus cannot be stunned. They don't bleed or feel fatigue or other acute wound effects, so they can keep on fighting for a very long time after your military has crumbled. Also projectiles and weapons don't get stuck in them, so piercing weapons like spears are not nearly as effective as against creatures with internal organs. On top of this they like fighting, tend to be reckless, and their blows cause burn damage. Their only poor statistic is that their limbs sever when broken, which means that they are susceptible towards high-damage dealing weapons and being overwhelmed by sheer numbers.
Easiest way to kill them is from far away with crossbows, ballistas, and catapults. The only problem is that they can hurl fireballs back, so either you need experienced marksdwarves, with surprise and in mass, or siege-weapon crews, and still you can risk ending up having charred dwarves if you're not careful.
The other method is fighting them with traps. Submerging them in water is unlikely to have much of an effect, as their innate heat will simply cause the water to boil away. A cave-in trap, however, will make short work of them.
[CREATURE:ELEMENTMAN_FIRE] [NAME:fire man:fire men:fire man] [TILE:'M'][COLOR:4:0:1] [LARGE_ROAMING][BIOME:SUBTERRANEAN_LAVA][FREQUENCY:1][DIFFICULTY:2] [MODVALUE:5] [FIREBREATH][FIREIMMUNE][LIKES_FIGHTING][MAGMA_VISION] [GENPOWER:5] [CANOPENDOORS] [NOT_BUTCHERABLE] [NOPAIN][EXTRAVISION][NOBREATHE][NOBLEED][NOSTUN][NONAUSEA][NOEMOTION] [NOSTUCKINS][SEVERONBREAKS][NOSKULL][NOSKIN][NOBONES][NOMEAT][NOTHOUGHT][NOEXERT] [BUILDINGDESTROYER:2] [NOFEAR] [PREFSTRING:licks of fire] [ITEMCORPSE:BAR:NO_SUBTYPE:ASH:NO_MATGLOSS] [NOSMELLYROT] [BODY:HUMANOID_SIMPLE] [SIZE:7] [ATTACK:MAIN:BYTYPE:GRASP:punch:punches:1:6:BURN][ATTACKFLAG_WITH] [NO_DRINK][NO_EAT][NO_SLEEP] [ALL_ACTIVE] [NO_GENDER] [FIXED_TEMP:10800] [SWIMS_INNATE][SWIM_SPEED:2500] [POPULATION_NUMBER:15:30] [LARGE_PREDATOR] [CANNOT_UNDEAD]