what are the demon's fireballs made of?

They are mostly solid molten lava at the core, but so intensely hot that they emit a circular charge of fire, thus appearing as fireballs.

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Rubbing alcohol.

They are made by digitized mathematical algorithms.

Smaller fireballs.

I red somewhere that cacodemons spits some toxic ball made of gas and imps throw fireballs.

SFoZ911 said:

The mancubus obviously shoots actuall fireballs(fuel tanks on his back, cybernetic launcher...)

Hell nobles throws green plasma IIRC, i have read about it somewhere.

when i found the bfg for the first time, i guessed the bfg ball was some technical imitation of the baron's projectiles, since they had both that bright green.

the imitation turned out to be a lot more deadly :p