Cool.
As was mentioned actually just moments ago in another topic, 炎’s reading of ほのお comes from the meaning 火の穂 (head of fire). So the tip, the end of a fire. That’s the etymological origin. ほ is a reading for 火 you probably haven’t seen yet, and 穂 is pronounced and written ほ now, but in classical Japanese it was written ほ and sounded like お. So, it was spelled ほのほ but would have been pronounced ほのお.