Sandstorm says the following on fire resistance, in the context of environmental danger:
A character with a spell or effect granting resistance to fire applies this resistance to both lethal and nonlethal damage from hot temperatures.
Since someone with a ring of minor fire resistance has resistance to fire 10, and no temperature band is able to deal more than 10 nonlethal damage, and nonlethal damage from heat exposure is what causes heatstroke, that person is immune to heatstroke and the corresponding fatigue as long as they keep their ring on. (Maybe the exact timing of things isn't specified for if they take the ring off, but I don't particularly care about that)
That's all well and good, but then the very next section in Sandstorm covers dehydration.
... the creature must make a successful Constitution check each hour (DC 10, +1 for each previous check) or take 1d6 points of nonlethal damage. In particularly hot environments (those above 90° F), the time a creature can go without water before making Constitution checks is reduced
Nothing in there makes any mention of fire resistance having any impact on the amount of time before dehydration checks start to take place. It also doesn't say that the damage is "from hot temperatures", which is what the first quote said fire resistance reduces.
Same deal with sunburn:
If a character is caught out in the sun and completely unprotected ... the character is mildly sunburned and takes 1 point of nonlethal damage. After 3 hours more exposure, the character develops severe sunburn and immediately takes 2d6 points of nonlethal damage and a -2 penalty on Fortitude saves to avoid damage or fatigue from heat dangers until the nonlethal damage is healed.
The damage here isn't "from hot temperatures" either. And even if it were, the penalty on Fortitude saves isn't even "damage". So is this to say that our scaley fiend once again suffers sunburn just as quickly and just as severely as Mr. Human, or at least some reduced form of severe sunburn, despite being able to withstand the temperatures of the literal surface of the sun?