You might be surprised, 5582270. I thought it over before, and my conclusion is the obvious Greek Fire, the weapon the Eastern Roman Empire used in their naval battles. Greek Fire works more or less no different than our modern counterpart. Though nowadays we use gas as fuel, some still use a liquid mixture, a flammable liquid mixture.

Lighting it up as it exits the barrel is a task easy, but to project it far enough is the opposite matter. The Byzantines made a very efficient pump just for the purpose of sending the fires far enough to hit the ships. Greek Fire sticks, and even burns on water, unsurprising since the mixture itself is essentially oily. On the artwork for the Firespitters, we only see one side of it (The barrel side). I take that the other side, the side with the barrel with a dragon-shaped head, there is the pump mechanism attached to the strap (what the Firespitter is holding) connected to the fuel source (The Barrel).

And since Earthborn has an expertise in mechanical engineering (Guns, Tanks, Helicopters), a simple, albeit powerful yet compact, pump shouldn't be out of their league.

Oh, and for the mixture itself, I'll assume that saying it's nothing outlandish and/or magical won't do. Hmm... ...what if mixtures varied? I take that the mixture is essentially distilled oil combined with whatever you want to make it much more flammable. They even put resin in to make it sticky! The Wikipedia says that the following are possible combinations: Pine Resin, Naphtha, Quicklime, Calcium Phosphide, Sulfur, Niter (Saltpeter). I suggest that you look into that yourself.

On a personal note, my headcanon is that Salty Rock, the settlement near Dodge Junction, is where Earthborn mines most of their Saltpeter (Salt Rocks).

5584112 thanks, in my notes im trying to be more detailed then the basic descriptions that Poor Yoricks art shows, but fire spitters have very little to explain really other then the way the weapon works. I mean theres only so much you can do with a close range flamethrower XD devestating as they are. So explaining how the weapon works is a good way to make them feel more unique then just the go to flamethrower unit you see everywhere.

Has it ever been discussed if there are Unicorns actually loyal to Earthborn?

I always take the 'anything is possible' route of thinking, 5928985. Loyal Unicorns are certain to exist within Earthborn, properly dehorned, of course. I believe that a better question would be the following. We know that Earthborn hates Unicorns due to their great natural aptitude to sorcery which spells disaster for irresponsible ones. Their solution is to dehorn them. Without their horns, they won't be able to use their magic. Boom. Problem solved. But what happens next to the dehorned? Do they suffer discrimination still? If so, to how much degree?

I find that unlikely, however, since there were Unicorns who voluntarily chose to dehorn themselves and stay with Earthborn. This means that there ARE loyal Unicorns. What do they do after they are dehorned though? I have a couple of ideas:

Suppose that Unicorns are naturally more intelligent than the other two subraces, this will make them ideal for intellectual work such as researchers or engineers. This, however, will be saying that the majority (at least in terms of capability) of Earthborn engineers, their very specialty, is made up of dehorned Unicorn; unless they are majorly employed in other tasks which do not require tiring manual labor which is more fitting to Earth ponies such as secretarial paperwork (a bloody nightmare is what I would use to describe it).

My second thought is that dehorning a Unicorn means to slowly kill him. For some reasons, their horn is vital to their livelihood, and they steadily become weak and feeble, and eventually die seemingly naturally. Quite morbid.

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I feel they would still mostly suffer suspicion and discrimination. Forcing them to move to isolated villages and communities.

Since it seems like to a degree the Earth Pony tension with the Unicorns is also their reputation as Snobby-Elitist-Nobles, the Canterlot Elite. Would this tension eventually disappear with an Earthborn Victory in the AU? Hard to say but it would certainly lead to generations of inequality for the Unicorns.

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...Generations?
That's an odd way of putting it considering that this AU's present timeline is only approximately 15 years away from the canon timeline, though I concur that the number is enough to accommodate one generation. This particular generation were foals raised with the anarchic environment of this AU.

Now what does this have to do with discrimination? Along the way to the 15-year mark, Earthborn passed their law concerning dehornification after discovered Moon and Star's usage of slavery through battle. Now remember that at first Earthborn was only a separatist entity denying the rule of the Celestial Sisters. Unicorns shouldn't be discriminated back then. On the other hand, there's also the possibility that the movement for racial superiority back in Canterlot to reach as far as the Frontier, which would inherently cause discrimination. One bad apple spoils the bunch, I say.