Living Fire Blocks have a 1/50 (2%) chance of being dropped by any enemy killed in the Underworld during Hardmode, including the Wall of Flesh, and drop in stacks of 20–50 at a time.
What does living Fire Block do?
Living Fire Blocks are craftable Hardmode furniture blocks that, when placed, resemble small animated flames and emit a faint light. They deal no damage on contact, and can be broken with any pickaxe. ... Like other furniture objects, Living Fire Blocks have no collision, allowing the player to walk directly through them.