That was a bit confusing since other games I usually play threaten fire, cold, etc as other types of damage, ie: elemental damage. It seems I was playing in Very Hard mode in my solo campaign.
There is another question that emerged from my latest session. Can enemies NPC double damage or ignore armor from rolling a dragon in attack checks as PCs?
And sorry if I am not editing the file but it feels like I might break something there. Thanks for the clarification.
Fire damage is fire damage and that can have an effect on some creatures (some are immune to fire or take extra damage, etc.) but fire is also physical, the flames physically strikes you and touches you and therefore any armour you have will protect you against the flames.
And as was mentioned above, NPCs follows all the standard rules for the game and that means that any 1 or 20 rolled for them will affect the outcome of their skill rolls.