Lightning can start it, so can man or his creations. Fires will grow in size (possibly quite fast under the right conditions) and eventually burn lots of things in the room, including you! The fire will even spread if left un-contained.
But I've never seen fire in Withmore!
Our excuse has always been, its a big building, fire is a huge problem, so there's automated suppression systems everywhere. Those of you who already stink of rotten eggs know, this is now coded. Every indoor location (indoor being a relative term within the dome itself) has automatic fire detection and suppression systems. Keep in mind, these vary in sensitivity and power. In a lot of circumstances, you're going to have to get involved to extinguish a fire, but the system will help keep it something you can extinguish.
Oh my gawd, it got so big, I can't battle it!
Call 911, it might help, even if you're out on the streets of red. Going to battle something off the grid? Get some firefighter turnout gear so you -can- battle the big fires.
Fire! Cool, I like to play with Fire! There are plenty of ways to start a fire now. I'm not going to elaborate because it doesn't help anyone if we all run around burning the MOO down to experiment. You'll be persistent, you'll find them.