What kind of paint to use on a metal firebox in a wood burning fireplace?

I would say no. Auto paint is usually designed for cars that idle the majority of the time but only need to rush like a bat out of hell when it’s time to move. The materials in automotive paint are all oil-based, and they will release an odourous gas called nitrous oxide (or “silent killer”), which can cause headaches, nosebleeds or respiratory tract infections when exposed over an extended period of time because the gas itself smells terrible combined with the chemical reactions in your body that occur when you’re exposed to it long-term. For this reason alone I would say no.”
Also individual individual states may prohibit auto paint on their particular stove because certain chemical reactions within stove flames can react differently.