A moratorium is in effect on adding information from Fire and Blood until December 20th.
Firewyrms are creatures that breathe fire but have no wings, and are possibly related to dragons. They can bore through rock, soil, and stone. According to old tales, firewyrms were in the Fourteen Flames of the Valyrian peninsula even before the dragons came. Their young are not much bigger than a child's arm, but as they age they can grow to immense sizes. They have no love for men.
Firewyrms were encountered by slaves of the Valyrian Freehold in the mines of the Fourteen Flames. Burnt and blackened corpses were often found in shafts where the rocks were cracked or full of holes.
Septon Barth's book, Dragons, Wyrms, and Wyverns: Their Unnatural History, discusses firewyrms.