From the Super Mario Wiki
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Pyro Boxes are indestructible diamond-shaped objects with a hole in the center. They appear in all of Bowser's levels, as well as Lethal Lava Land and Rainbow Ride. About every three seconds, a Pyro Box shoots a stream of fire from its center. If hit by the flames, Mario will lose three life points and run around uncontrollably, possibly forcing the plumber over the edge of a cliff. If Mario jumps around while he's on fire, he may only take two points of damage.