Like a phoenix, a firebird is a bird that is perpetually on fire, providing it with a natural attack and defense superior to that of more mundane fowl.
Real World References
Firebird was an automobile released by Pontiac between 1967 and 2002.
It was also the original name of FireFox web-browser, the twin of the ThunderBird Email-Thingy
The Firebird appears in real world Russian folk tales and is the subject of a Stravinsky ballet.
Being 'birds' they fit the theme of Transylvito units being slang terms for women (along with Goyles, Dolls, and Molls.)