FAR 23.1182
Components, lines, and fittings, except those subject to the provisions of §23.1351(e), located behind the engine-compartment firewall must be constructed of such materials and located at such distances from the firewall that they will not suffer damage sufficient to endanger the airplane if a portion of the engine side of the firewall is subjected to a flame temperature of not less than 2000 °F for 15 minutes.
§23.1351(e)
(e) Fire resistance. Electrical equipment must be so designed and installed that in the event of a fire in the engine compartment, during which the surface of the firewall adjacent to the fire is heated to 2,000 °F for 5 minutes or to a lesser temperature substantiated by the applicant, the equipment essential to continued safe operation and located behind the firewall will function satisfactorily and will not create an additional fire hazard.
FAR 23.1191
Too long to copy here, but says that the firewall must be fireproof, made from certain materials, or proven acceptable by testing similar to Dan's test rig.
§23.1193 Cowling and nacelle.
(d) Each part behind an opening in the engine compartment cowling must be at least fire resistant for a distance of at least 24 inches aft of the opening.
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None of these specifically talk about the occupant, or the upholstery, or the pilot's footwear. If you're saying that I can't have fire resistant black foam on the inside of my firewall, I don't see it here. And §23.1193 (d) Cowling and nacelle doesn't apply to the inside of the aircraft unless you plan to leave a hole in the firewall into the cockpit.