From the Super Mario Wiki
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Bonfire Blocks (sometimes referred to as Fire Blocks) are special types of blocks that show up in every title of the Wario Land series, starting with Virtual Boy Wario Land. They appear as red blocks with a yellow border and a picture of a flame on them. In Wario Land II, they have a basic, red and yellow block appearance, in Wario Land 3 they are purple blocks, while in Virtual Boy Wario Land and Wario Land 4 they are made to resemble cubic stones. In Wario Land: Shake It!, they appear to be made of metal.
Bonfire Blocks are completely impenetrable and, in most of the games, they can only be destroyed by having Wario turn into Flaming Wario and touch the block. On the other hand, in the first game they ever appeared in, they can be broken only by the flaming power of either Sea Dragon or King Dragon Wario. Bonfire Blocks are usually placed in strategic places and often hide treasure and coins; they can also cover Warp Pipes.