The crackle, the flicker, the scent of smoke and wood, the glowing warmth that caresses away chills. It's hypnotic, and on such a primal level that we don't always notice it.
Staring into a fire, just in the moment is meditative. Your mind can either roam free, or empty itself out to let you float in the now.
I don't see why autism would have anything to do with it. People have been sitting and staring at fires for thousands of years. It's hypnotic, and it's a good kind of meditative clear your mind kind of situation, or good time to just sit and think.
When I was a little kid, if I was good then my grandparents would fill a big pot with torn-up newspapers, set them on fire, and let me watch them burn. Apparently this isn't a normal child-rearing practice. I'm not sure why.