It is a hardwood hence is moderately good for firewood. When seasoned properly, it splits easily, burns relatively clean and produces heat in fair amounts. There can be difficulty in splitting the wood if the seasoning is not well done.
Cottonwood is safe to burn in a fireplace or woodstove, but it is not a good wood for keeping your home warm. If you just want to burn a type of wood to create a cozy atmosphere, this one would be fine, but I would go with a different type of wood if your goal is to create warmth.
Cottonwood is definitely not the best wood to burn if you want to produce a heat source. Ash is considered to be the best wood for burning because it is a good heat source and produces a constant flame. Beech burns similarly to ash. So, if you are looking for something to heat your home, I would suggest one of these woods instead.