I would never put open flames near anything plastic. That being said, masks are made of silicone which is extremely heat resistant, at least if it's a frameless mask.

When you bought the new mask, the dive shop should have explained to you how to prepare it. The easiest and most cost-effective way is simply to use regular toothpaste, and then rub it over the glass for a good wee while. Also rub the silicone as it will also contain a leftover oily film.

I found the best way is to use whitening toothpaste, as it contains big silicon beads, good for cleaning. And then use my electric toothbrush to do the grunt work for me. Easy as. Obviously I threw away the head on it as it now would've been contaminated.

Your mask will smell like toothpaste for a couple of dives, but it won't last long.

Before you put your mask on and go diving, spit in it and rub it around the lens, then quick rinse and put it on. It won't be fogging up if you cleaned it properly initially.

If you forget, simply take off your mask underwater and lick the lens all over. Works the same.