Thanks, Dug - I just looked it up.
It's still a little unclear. A phoenix (according to the myth) lives for 500 years, then perishes in its own flames, and out of its egg pops a new phoenix. A firebird flies in flame, and is also supposed to be immortal.
So ... they're both immortal and fiery. *shrug* The only difference seems to be that a phoenix isn't
always on fire.
Also, the firebird isn't just a Russian myth, but is also common across the Slavic world.
Can't someone write a new variant on this myth? How about a frostbird, one that's always covered in little icicles? Or a lightningbird, i.e. one that's always covered in little sparks of electricity? =P