In old beliefs the sun was a god and a mighty one. As many other gods, it lived in the sky. Of course, people living on Earth needed mediators to communicate with gods. Birds, which flew between the sky and Earth, became mediators. There were such birds in real life but in fairy tales they were magical, and such was the Firebird with wings of golden flames.
The Firebird is also associated with happiness. One tries to catch it but fails. It flies away leaving in one's hand nothing but a bright feather, hope.
(Written and translated into English by Vadim Shchanitsyn)