In this answer I will relate fire to the combustion of fossil fuels, coal and gas. Thus the gas is burned in oxygen to produce carbon dioxide and water, this process gives out a lot of energy, this energy can be seen as light and felt as heat. This reaction needs a flame to begin at room temperature although at greater temperatures it can happen spontaneously, once the reaction has begum the heat produced is enough to continue the reaction until there is no more fuel or no more oxygen. So to consider fire at a molecular level, the fuel can be a solid, liquid or gas, consider coal, petrol and a cooking flame. The flame seen is comprised of the gases escaping, the carbon dioxide and the steam.
All visible matter including fire is comprised of molecules.