Fire Demons are a race of dark magma-like creatures that live on Muspelheim. They were led and ruled by their lord Surtur.
Thor was caught by the Fire Demons Lord, Surtur, in Muspelheim. Thor managed to escape with the help of Mjølnir, so Surtur summoned his Fire Demons to kill him. Thor defeated the demons, killed Surtur and took his crown back to Asgard. A few days later, Loki put the crown in the Eternal Flame in order to resurrect Surtur so he would cause Ragnarök to kill Hela.
The Fire Demons are loosely based on the Fire Giants of Norse Mythology. In the comics, they were the offspring of Surtur and entirely made of flames.