Fire resistant clothing is already available.  Hikers who build fires judiciously will use non-sparking wood, which I believe means avoiding most coniferous fuels.  Fire retardant sprays for fabric are available:  

A better way to stay warm is to carry adequate insulation and eat sufficiently so as to maintain your metabolism.  A good many folks who depend upon wood for cooking use packable stoves, which diminish sparks and embers considerably.

Very honestly, i don't think this is much of a problem.  i can't recall the last time I caught an ember, but it left a tiny scar, which vanished on the next laundering.  When i light a wood fire these days, it is some sort of emergency, and I am not at all concerned with incidental dings to my clothing.

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