The Firebird appears to be a giant eagle with huge wings and its whole body is made entirely out of lava. He also has glowing yellow eyes. He is animated by well-known animator John Pomeroy.
The Firebird has no alliances; he serves no one, he cannot distinguish friend from foe - he is a primal living force of destruction. He lives only to destroy and despises all that brings life and creation (The Spring Sprite). His only weakness is that he only destroys - he cannot create nor can he persist, as he returned to his slumber when there is nothing left for him to destroy.
This interpretation of the Firebird actually conflicts with the original version of Igor Stravinsky's ballet. In it, he was described as a benevolent, peaceful being who bore no traits of evil. However, Disney's version was violent, destructive, and merciless, showing no qualms of destroying a newly created forest.
When the Firebird was accidentlly awoken from his hibernation state by the Spring Sprite, when she touched the stone on his volcano for curiosity, he unleashed a brutal and terrifying display of its destructive power on the Sprite's and Elk's forest. It is unknown why he attacked the former to begin with, but it can be assumed that he was enraged for being awakened, or that he simply felt the urge to begin a chaotic rampage. He pursued the Sprite until she climbed on one big tree. The evil phoenix then gave her a final hit that turned her into a tiny, helpless creature. After the destruction to the forest was done, he returned to his slumber.
If you look closely while the Firebird's awakening, you can see evil spirits, demons and lost souls swarming around.
He shares many similarities to Chernabog, the villain from Night on Bald Mountain in the original film. They both emerge from a mountain, they reek chaos, they can both manipulate fire, they are both forces of destruction, and good ultimately triumphed over both of them, although the Firebird is not necessarily evil like Chernabog. It is interesting to note that they both appeared in the last segment of the two movies. However Chernabog is a force of evil while the Firebird is a force of nature. Another difference between them is despite the fact that the Sprite restores life to the forest, the Firebird leaves on its own terms, whereas Chernabog is forced to cease his evil antics by a holy bell.
He is the only character from Igor Stravinsky's ballet to appear in the segment.
Yveltal, the Destruction Pokémon from Pokémon X and Y Versions, bears a strong resemblance to the Firebird. Coincidentally, enough, Xerneas from the same game resembles the Elk.
In Once Upon a Time, the firebird is only mentioned by Henry when he choose the name of Emma's new operation to rescue Hook in Hadès' world.