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.
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.
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.