My coat is a Marmot but I was dumb. It was ultralight at the time I bought it, and the outer fabric is definitely highly meltable.

We were burning invasive brush in snow last March and the embers flew all over me. Now I wear a thick thrifted flannel over it. Or I wear my Fjallraven waxed canvas jacket.

I work with textiles, and don't think you can get a fireproof non-melting ultralight shell on down - but I could be wrong.