I really don't remember (+quick check) Firebird for the email client, do you have source for this?
No, the email client was never called Firebird, that isn't what I was saying.
Browser: Phoenix -> Firebird -> Firefox Mail client: Minotaur -> Thunderbird
Firebird was the browser's name, after phoenix and before rebranding to firefox.
Predecessor of Firefox was Firebird, and before that it was even called Phoenix.
https://en.wikipedia.org/wiki/Firefox#Name_changes