This should explain it.
http://www.killermovies.com/forums/misc.php?s=&action=bbcode#imgcode
Namor's fire resistance is very inconsistent. His best showing vs fire was when it took THREE Human Torches to KO him via one HUGE combined fire attack.
Namor should win anyway. He's fast and even using some of his low showings vs fire attacks, he still has enough resistance to get to Pyro and destroy him before being taken down by the heat.