A fire is not a substance, it is a process going on. SOMETHING reacts with oxygen, and that produces heat and SOMETHING-oxide. There is a wide variety of possible SOMETHINGs, including even metals if you have the right condition. For example methane (CH4) burns with such a formula:

CH4 + 2O2 => CO2 + 2H2O

(all numbers that are after letters should be subscript).

So the elements involved into the methane fire are carbon, hydrogen and oxygen, the substances - methane and oxygen incoming, water and carbon dioxide outgoing.