Windwalker works. The other three don't quite play in the way I'd hope - Heartwarder is interesting, but isn't quite the image I'd had in mind for the Firebird - less a sensualist, more of a maiden goddess. Elemental Archon is far too flashy, although it might be useful for a villain (blame him!) Silver Star muddies the waters; rangers shapechange naturally in this setting, and I wasn't intending to use lycanthropes at all just to avoid confusion. The whole sourcebook looks neat, though, I'll read through the PrCs and see if there are other possibles.