Okay. Ford "invents" the Mustang. Its a phenominal hit. It is a pony car. GM introduces the F body to compete with the Mustang. Use some simple logic and decide what your Firebird was intended to be.
Certainly, the phrase or label was promoted by Ford. They INVENTED the class. They gave birth to it. They named it. And thank God they did. Otherwise we'd all be on the Mustang forum right now.
I'm a hobbyist. Not a professional. Don't be hatin'!