The Fire Demons are natives to the realm of Muspelheim, one of the Nine Worlds of Norse mythology conquered by Odin. The species are ferocious due to their harsh environment they are subjected to live, and sometimes venture to the other nine worlds in the name of war. They are nomadic race within their lands.
The Fire Demons are the offspring of the great fire demon Surtur, whose existence is said to precede that of the Gods. The Demons' appearance and height varies but tend to be about the same stature as the Gods of Asgard.
Marvel Cinematic Universe
The Fire Demons are minor antagonists in the 2017 Marvel film Thor: Ragnarok. During his unsuccessful quest to search for the Infinity Stones, Thor was captured by Surtur, who gloats of the impending event known as Ragnarok that will spell Asgard's doom. Unperturbed by this, Thor frees himself, prompting an angry Surtur to summons the Fire Demons (and the Fire Dragon to kill Thor. However, using his hammer, Thor manages to defeat many of the Fire Demons before taking down Surtur and claming his crown.
Witnessing this, several more Fire Demons tried to take Thor down to avenge their master's death, but Thor flies away, delivering Surtur's crown to Asgard for safekeeping. However, the arrival and rampage of Hela forced Thor and Loki to use the Eternal Flame and the crown to revive Surtur back to life, knowing that Surtur is the only one who can defeat Hela. As Thor and Loki manage to evacuate the Asgardians on one of the Grandmaster's ships, Surtur uses his flaming sword to destroy Asgard as foretold, killing himself, Hela and her Berserker Army in the process.
It is unknown what happened to the Fire Demons following the death of their master and the destruction of Asgard.