Whatever coating you use on your firewall, it must not be combustible or support combustion.
That's why it is called a firewall.
The nearest heat resistant paint coating to a ceramic, vitreous enamel is " Sperex " exhaust paint.
However, after it has dried it needs to be cured at exhaust temperatures for it to become inert to 1200 degrees.
Thus, unless the whole firewall is completely separated from the aircraft, curing it with a blowlamp is a no brainer.
Also, until Sperex is properly cured, it is easily dissolved with petrol( Gas ) and other solvents.
Sperex also messes up very easily.
So unless something else is available, keep the firewall clean and keep the engine cowlings on.
A polished firewall will reflect heat and will keep everything behind it just that little bit cooler.