14 CFR § 125.145 - Firewall construction.
Each firewall and shroud must -
(a) Be so made that no hazardous quantity of air, fluids, or flame can pass from the engine compartment to other parts of the airplane;
(b) Have all openings in the firewall or shroud sealed with close-fitting fireproof grommets, bushings, or firewall fittings;
(c) Be made of fireproof material; and
(d) Be protected against corrosion.