Originally called Phoenix, since it was Netscape Navigator, reborn.
But Phoenix Technologies disliked that, so they renamed to a descriptive name for the same immortal bird – Firebird.
The Firebird database people would have none of that, so after a few-months gap between 0.x releases, they found the closest thing they possibly could which was not trademarked. It had nothing to do with the original name idea, fire being a weak link.
And we’ve been stuck with that stupid name for two decades.