There is no standard meaning because it is not a real thing. I imagine that the person was wearing a medal or medallion around their neck which had the shape of a bird with a fiery colour. There is a piece of music called "the Firebird" by Stravinsky which I think is based on a Russian folk tale. You could search for it as this may be the origin of the firebird mentioned in your text.

The only way to find it out is to read the novel, which is bound to tell you more about it sooner or later. "A Thousand Pieces of You" is a science fiction or fantasy novel. From a Google search I found a link to an excerpt. http://readingunderground.org/2015/03/24/teaser-tuesday-30/ "The Firebird" is capitalized. It is a proper name. "The Firebird" is some kind of device, charm, or amulet that can carry the bearer "across dimensions" into other parallel universes. There's no doubt in my mind that it's supposed to evoke the image of the mythological bird. If nothing else, it calls to mind the Pontiac sportscar which featured an image of a "firebird" on its hood. The cross-dimensional transport gadget probably looks something like this: http://images.highperformancepontiac.com/images/0605_hppp_06z+2000_pontiac_trans_am+hood.jpg

This is hard to answer without knowing more about the book. I cannot understand it just from this small part.

not **

Sorry A thousand pieces no peaces ^^"