For resistance to fire:
It's pretty straightforward to use stunts and extras.
Because I am a dragon, I have armor:2 against heat-based attacks.
or
Because I am a dragon, I have +2 to defence rolls against fire.
If you go this route, then stress and consequences you do take will probably represent an attack finally penetrating your resistance. This is no different than using a Kevlar vest with armor:2 to reduce stress from a bullet, or a bonus to Physique defence representing your ability to get punched and keep on coming.
But what about immunity to fire, you ask?
Here's the deal: Aspects can make you immune to fire, but they probably shouldn't make you immune to stress from fire attacks. It's a matter of exploiting the fact that stress is not damage, and consequences aren't damage either (though they can be). (For an experimental way to represent immunity through stunts/extras, see this answer.)
Stress is "the ephemeral toll of participating in a conflict," while consequences are "injury and trauma you just can't shake off after the dust settles" (emphasis mine).
Thus if you're immune to fire, taking stress from a fire attack represents your ability to keep the fire in the scene from impacting the narrative in ways you don't like. Once you start taking consequences, that means you didn't prevent the fire attacks from doing bad things which cause you lasting trauma or drama: Accused of arson is just as valid a lasting consequence as Covered in burns, and probably more leads to more interesting story.
If this seems esoteric or weird--it kinda is!
Stress and consequences have spawned several questions about their relationship to hit points (or lack thereof). To try and wrap our head around it in this context, let's look at an extreme example of immunity to damage: Superman.
We shouldn't just give him a high rank in Physique and call it a day, because Superman isn't just good at avoiding damage: most of the time he actually can't get hurt even if he wanted to, and skill rolls are set up to fail regularly. We could give him a stunt or an extra to make him immune to physical stress, but that seems dishonest to the character concept: Supes gets thrown around, punched through buildings, and thrown into space. He's vulnerable to physical stress, he just doesn't bruise or bleed because of it.
But with an aspect like The last Kryptonian, Superman is narratively immune to nearly all physical damage.
So Supes takes physical stress, sure. But stress is not damage! It's his ability to stay in the fight without lasting consequences: he absorbs or redirects the attack safely. When he can't use a stress track to absorb the shifts, he takes lasting consequences if he wants to stay in the fight, but consequences don't have to be damage either. When Superman is unable to handle a physical attack, it's almost always shown as being redirected into the environment: he's unable to control the force being leveraged at him.
This tells us that when Superman takes a consequence from physical stress, it probably represents collateral damage (Injured child or Jimmy's been shot!). If instead he chooses to be taken out by physical stress--or to concede in a physical conflict--it means he let the villain succeed rather than have innocent bystanders get hurt (Luthor gets away while Superman holds the falling build up long enough for Lois to crawl to safety).
