From the Super Mario Wiki
Bonfire Blocks are special types of blocks that have so far only appeared in the Wario Land series, namely in Wario Land II, Wario Land 3, Wario Land 4 and Wario Land: Shake It. They appear as red blocks with a yellow border and a picture of a flame imprinted on them. In the first two games, they have a basic block appearance, while the Wario Land 4 incarnation took on an appearance resembling a rock.
In Wario Land: Shake It!, they appear to be made of metal. These blocks are impenetrable; not even a bomb can break it. The only way to destroy a Bonfire Block is by having Wario turn into Flaming Wario and make contact with the block. Bonfire Blocks are usually placed in strategic places and often hide treasure, coins, or a Warp Pipe. A similar block is the Snowman Block, which will break when made contact with Snowman Wario.