Gelatin is the main ingredient that makes marshmallows firm instead of soft like marshmallow fluff. However, gelatin is safe for dogs to eat and can even be beneficial to dogs' health. This is not a dangerous ingredient. There is no reason dogs shouldn't eat marshmallow fluff, as long as the above-mentioned harmful ingredients are removed. Be careful with marshmallow fluff purchased at the store. These products are often high in artificial additives and sugar.