The Firebird is the alternate form of Emily Hayes when she loses control of the stone.
After Emily lost control of the stone, it transformed her into a giant firebird. Emily soon destroyed the underwater fortress and attempts to kill Trellis multiple times, and flies off. As the fire phoenix soars in the night sky, meanwhile within her subconscious, The Voice had begun to manipulate her mind.
Her entire body is purely fire in the shape of a magnificent phoenix, the bird made of burning fire-that she burned Trellis with when Emily was not in the lead of the amulet. She has piercing eyes of flames.
When Emily lost control she was turned into a Firebird like the one she made with Vigo Light-another stone keeper. They were sending a message to fallen stone keepers at the end of, Amulet book four-The Last Council. It seems like only the human species turn into elemental phoenixes when the lose control over the amulet's powers. In the stories told of Leon Redbread about four stone keepers losing control, Leon goes on with the story of a young elf who was one of the stone keepers, who later turned into the elf king. Every time an elf loses control thy turn into a giant version of themself. But when Emily lost control she became a Fire Phoenix.
Human Stonekeepers/ Fire Phoenix
It is not specified that a Human Stonekeeper can or can't turn into a giant version of themselves. Many times it is mentioned that a Human Stonekeeper has lost control but there is never a flashback, nor is it mentioned what happened or happens afterward.
Flight - With wings comprised of flames, she is seen to be able to fly.
Eternal Fire- Emily was seen flying out of the deeps of the ocean with out burning out.
Fire Throwing- The fire phoenix can release a piece of fire form to hit something with it, like how Emily did to Trellis in "Firelight".
She is seen to burn anything she touches, as seen in Supernova.