Watch what happened to Janus here
....and over the next couple of comics. I had previously assumed all the fire was due to it being a fire everlasting, maybe it happens each time.
Or, since that everlasting lost his head at about the same time Guy "died" and got HIS power... it was granted by the same god/goddess... and the power merely manifested itself differently. Fire=Rage.
We'll find out soon I'd wager.