” A fire (burning) uses no mass. “
Uh, Again .... E=MC^2 is universal.
Einstein formulated it BEFORE we knew about nuclear reactions. It applies to ALL Energy transformations.
Unless we are accelerating everything we need for releasing energy, E=mc2 has zero to do with burning.
Einstein said if you accelerate mass to the speed of light, it will convert completely to energy. That’s what the C is doing in that equation. I don’t accelerate my firewood to the speed of light to cook food at camp.
I just shoot a half-can of lighter fluid at it and then toss in a road flare.
You can tell breakfast is ready by the mushroom cloud coming from my camp. They guy with no hair on his face is the chef.