The Lesser and Greater fireflies (in-game names Pikkutulikärpänen and Suurtulikärpänen) are flying enemies made out of Volcanic Rock that spawn in the Coal Pits, either naturally or from a hive hanging from the ceiling.
Fireflies shoot a projectile resembling the Firebomb spell, which expires after a short time or on contact. Like many enemies, their projectiles can be effectively dodged by moving vertically. Staying off the ground also helps with avoiding the Lava they bleed when damaged. They will also charge at any foes which get too close, dealing melee damage.
Like other fire-based enemies, they are vulnerable to water and will take damage from being submerged in even small amounts of water. If they become Wet, they will die shortly thereafter due to taking continuous damage from the status.
The English translation for the enemy's name, "Firefly" may be a pun based off of the fire and insect-like traits the enemy itself has.