Firebars are a type of obstacle that has a bar of fire that emits from a block. The bar then rotates around the block and will harm Mario if touched.
The Firebars always appear in the Castle levels of this game. There are usually short ones and longer ones and Firebars mostly appear near other hazards such as bottomless pits and near enemies such as Podoboos. Mario cannot defeat them by any means and must avoid them.
Firebars are only seen in Lethal Lava Land in this game. Their purpose is exactly the same as in Super Mario Bros. except they spin around in circles on the ground.
Firebars appear only in Koopa Bros. Fortress and Mt. Lavalava. This is the first game where Firebars can be defeated and Mario can only defeat them by jumping over them many times. Note that the Firebars spin faster and faster after each jump and after the last jump, they burst apart into many coins.
Firebars only appear in the Palace of Shadow in this game for Mario. Mario can jump over them to avoid them. They also appear in the Castle level of Bowser's last level where he must avoid them. Due to his large size, it can be pretty hard to avoid them without taking damage.