SPIRIT?? You're insane just fire & smoke no suggestion or even hint of any such spirit escaping the home wow you're really stretching there.
There is a woman that walks out the burning house, pauses, and then exits left behind the fire truck. Blink and you'd miss it.
_
Every person that served can be called a veteran, but not every veteran can be called a Marine.

by all means, articulate what you think about it then

You got everything about this wrong
SPIRIT?? You're insane just fire & smoke no suggestion or even hint of any such spirit escaping the home wow you're really stretching there.
You didn't see the firemen and EMTs arrive, you don't know what they did first.

Yes I must say how the fire brigade works in walking pace is what seems awkward to me and made me come to this board in the first place.

A lot of strange stuff is going on in this very short sequence,
some already mentioned, besides the "mother" obviously.
Contrary to what some claim, it is definitely the same house,
seen from the side toward the rear.
The driveway out to the main road is to the left.
The lake is to the right.
There are three vehicles visible.
The large red fire truck is parallel parked in front,
facing back out toward the road,
as if it was turned around completely
before eventually stopping there.
The white ambulance is parked behind that truck,
perpendicular to the front of the house,
with its opened back doors facing in.
And the small red fire van is driving away,
but not toward the road. It's heading for the lake.
The firefighters seem strangely calm or even uninterested,
and it appears as if they have not yet begun to fight the fire.
Someone is just now opening the side equipment panels on the truck.
Others enter from the left side of the screen,
bringing fire hoses from somewhere, some stopping in the middle
and others exiting again on the right side of the screen,
dragging one of the hoses with them.
Still some others enter from the right side of the screen,
and someone seems to give a signal back toward the lake.
So is something happening behind the house, at or near the lake?

I've heard that theory, too that the house was their old house. I don't think I buy it, though.
Seems like a stretch. And also, why would they move to a new house, and then sell that one immediately? (don't forget the boys find the house for sale online.)
It's just too much of a jump to automatically assume that Elias burned the house down, only because the mother finds a lighter in his bed.

Valid points.
I think the movie's success lies in its ability to beget diverse polarized conclusions for almost each and every scene from a setting that appears surprisingly minimalist. Speaking of the house, i was reading an 'explanation' yesterday that even said that the burning house isn't the same house, and that its a previous residence of the characters. Let me share it, wonder how every one here will react to such s statement.
http://jarviscity.com/2015/08/20/goodnight-mommy-ending-explained/
I will be watching the film again solely to have a better look at that house.

first, who exactly called the firemen in the first place?
Well the house is pretty modern and clearly belongs to someone who has (had?) money to keep it, so it wouldn't surprise me that it had some sort of alarm and if a fire broke out the firemen central or something would immediately be notified and send some vehicles their away.
second, well i am not that smart and very fortunately never experienced such a scenario, but you don't take a body in an ambulance and keep the vehicle standing just still, even if the body has been burnt beyond recognition. There was an ambulance, and it was just standing there, what was it waiting for? which denotes in practical sense that there was no body inside. This brings me to my third point of confusion
Assuming the bodies (or 'body') are yet to be discovered, do the firemen look THAT alarmed? They are trying to take down the fire, but are they rushing in to save the people inside? Even if one completely presumes that every one inside is dead, in humanitarian terms can one be so cool as they appear to be?
There
could
have been someone inside the ambulance, maybe they were trying to perform CPR or some other urgent treatment. When it's a near-death scenario they have to do things on the spot and not while the ambulance is driving down the road as it makes things much more difficult. So IF someone was in the ambulance, they were likely trying to stabilizing them first, but I don't think so.
As we can see, the fire is still raging inside, and firemen have to play it cool (not a pun, unfortunately) because people's lives are on the line, they can't jut act on their own.
The fact that the "spirit" of the Mother leaves the house and not one of the vehicles could hint that her (dead) body is still in there, so must be the body of Elias. Or maybe they were already presumed dead at the scene and the firemen only had to put out the fire.
What's weird is that in the very beginning of that scene, you can see a firemen truck
leaving
, perhaps hinting at that they're not needed anymore, as in the situation is already almost over/under control? That's how it looks to me, they already know that no one survived so they're all just methodically putting out the fire.
Maybe some firemen did go in and quickly found the bodies, after all they were both in the living room, where the fire started. Well at least the Mother house, Elias must've collapsed somewhere nearby due to the smoke.
Fine, fine, I'll leave! But first I'm going to bother these peanuts! Hmm? Yes? Hmm? HMM?