Firewood is wood that has been cut or collected so that you can burn it — in a stove, a fireplace, a fire pit, or under a pot. It is not wood for building a house, and it is not a living tree: the whole point of the word is “this wood is for the fire.” People talk about chopping firewood, stacking it so it dries, and bringing it in before winter. Wet firewood smokes and will not catch easily; dry firewood is what you want. In Taiwan you meet it more at a campsite, at a 烤肉, or in an old countryside kitchen than in a city apartment. The word is simple, physical, and everyday wherever people still burn wood for heat or cooking.