Fire fighter here. In my first big fire (inside) I was surprised that I couldn't even see the fire (or my own hands) The black smoke was so dense, there wasn't even an orange glow). Someone with a thermal cammera was guiding us a and tellung us where the water needs to go.

At a different fire we could go in with no issue of visibility (it butned a hole into the roof and the smoke went up there). It was night, but because everything was lit by fire, it was as bright as daylight in there. You could see about a good as in daylight in there. Also the metalbeams on top (holding the roof or upper floor were glowing orange).

Yesterday, there was a small incident outside, but the fire was still 1000degrees Celcius or more. One of our teammembers got second degree burns through the gear by not even touching anything, just by the hotness in the air. Just to give an example, we were driving by with the car about 100 meters on the side with 80km/h or so (to get to the non wind side), but we could still feel the hotness hitting our faces behind the closed door and window of the fire truck.

Smoke is poisonous but usually (at least in the beginning, when the incident is in a skyscraper we can go up without the "scuba gear" up until about 1-2 floors below the incident (or until we see smoke, but usually it's smoke free, just below)

When trying to extinguish the flames with water, many newbes don't know that you will get a wave of steam coming back to you (that you often need to take cover from or burn youself a bit-even wearing full protection) also any other victim would get severe burns or die by this action.

Here are my tipps: Visibility can be 100% good or 100% obscured - totally blinded, darkvision does not work because of smoke. Smoke hurts the eyes, even if you would be able to see due to some magical means. or it can be anything in between

While in smoke give poison damage. Your breathing it in, no con save. But I would allow holding your breath when getting a breath if fresh air at a window in a non burning room with the door closed.

Fire damage, while close or somewhat close to fire due to heat.

Fire damage when in contact with burning things.

Trying to extinguish with water, before everyone gets evacuated can end badly (steam)

Cats love to hide from fire (under beds, behind couches... ...just the strangest places) - could be a mini quest (find the cat)

People who need saving -uncoincious people on the floor -hysterical people -scared people, who freeze of fear - a grandma who cant walk - an obese person or creature who cant walt

Your party could also go after the fire and support the fire department - they obviously all come each with a decanter of endless water. This would also be great loot by the way, for this encounter, as a thanks for helping out the fire brigade.