Do you construct them differently if you have a fire made vs if you dont?
Would you open up a shelter a little more to get the heat radiating to you, vs a really small entrance that you can cover up?
Couple of thoughts:
1) I would be leary of putting a fire too close to a debris shelter because of the fire hazard and the fact that I would also be using the fire to cook. Not a good idea to have food scents too close to sleeping area.
2) Having the fire a safe distance from the opening to the shelter with a good wind break might be advisable.
3) Heating rocks, or aluminum water bottle, enclosed candle, putting coals in a pot, etc might be options to warming the inside of a well constructed shelter.
Then again, I will readily admit that I've never spent the night in a debris shelter yet, so my tune may change as I freeze my tilly-tail off...