They stack.
Stacking these two spells to achieve 100+% fire resistance is one of the joys of tanking. Be warned that Resist Fire/Cold lasts for only a short time. I don't use the combo it unless I am planning a full-out fire bombardment (Sunfire, Flame Strike, and maybe Incendiary Cloud in addition to the ubiquitous Fireball), or unless I'm fighting cold-users and want a little extra protection.
I have Protection from Fire active at all times on my main tank(s).