Always wanted to draw some kind of giant animal screaming/screeching/roaring at Celestia. I think it's a thing. Also, the bird ain't on fire, forgot about that....well they don't literally need to be on fire to be a firebird right? All phoenixes are firebirds but not all firebirds are phoenixes.
Anyways, according to my head-canon, this happens before harmony/search for (the elements of) harmony. On another note, I really do love Fantasia 2000's Firebird Suite sequence.
Celestia, of course, rightfully belongs to

and

Edit: AHA! I knew I was forgetting something! I forgot to put celestia's wings! Be right back!
Edit 2: And back with wings!
Seeing as there's a seeming distinction between Firebird and Phoenix, and that this thing is twenty times the size of every Phoenix we've seen in the show, my personal view is that the Firebird is an even rarer, more powerful species of Phoenix, perhaps even a leader of sorts. Celestia had to confront it and reason with it over a millennium ago, in order to form an alliance between their two races, so that nesting ground could be established in Equestria in certain places :3
Anywho, very terrific job