The Firebird was a creature encountered by Jack Harkness. He mentioned it to Gwen Cooper when she asked him what was the most beautiful thing he had ever seen. According to him, it was very small, even smaller than a hummingbird. It was literally made of fire. It only lived for a minute. It blazed in different colours and sang. Jack spoke of an experience with the firebird:
It gets so bright you have to close your eyes. And when you open them, it's gone. But the image stays behind your eyelids for longer than it was alive.