I think it may be melee only... In D2:ED I don't recall ever having a bow with 3 enchantment slots, and normal damage / magical damage were always my first and second choices for weapons.
If you visit your enchanter, does Fire Curse show up if you select a bow? If so, maybe save, try the enchantment on a weak bow (to give more shots per opponent) test it out and if there are no visible signs of Fire Curse being triggered, check your combat log.
