Originally, it laid dormant inside a mountain in the middle of forest. When the sprite of spring noticed that it was unable to make anything grow on the mountain, it curiously explored the top of it and saw a mysterious stone in the certain which turned out to be the Firebird's beak. Upon touching it, the sprite's magic awoken it. In a rage, the Firebird rose up, transforming the mountain into a volcano and shot fireballs at the sprite who fled the mountain. The Firebird transformed into a river of lava. and pursues the sprite, destroying the forest in the process before presumably killing the sprite by devouring her. After its rampage, the Firebird returns to its volcano and returns to dormancy. In the end however, the sprite is shown alive and restores the forest to its natural beauty.
The story is an allegory of the cycle of life: birth, growth, death, rebirth; with the firebird representing the cycle of death-to-rebirth, the sprite growth, and the stag the masculine energy needed for birth.
In the original piece, The Firebird is gentle and actually pleads for mercy. The Firebird Suite segment is also very similiar to the "Night on Bald Mountain/Ave Maria" segment from the first Fantasia. It's were the first villain Chernabog appears.
It shares a far number of similarities to Chernabog, the villain from Night on Bald Mountain, and the Big Bad Wolf, another villain. They both appear in the final segments of each movie, both emerge from a mountain, they reek chaos, they can both manipulate fire, and good ultimately triumphed over both of them (although the Firebird was not essentially driven away as was Chernabog), but not the T.Rex. It is interesting to note that they both appeared in the last segment of the two movies. Considering the similarities it is possible that it might be one of Chernabog's creations.
This interpretation of the Firebird actually conflicts with the original version of Igor Stravinsky's ballet. In the ballet, the Firebird was described as a benevolent, peaceful being who bore no traits of evil. However, Disney's Firebird was violent, destructive and merciless, showing no qualms of destroying a newly created forest as it hunted the sprite.