I owned both mustangs and firebirds. Enjoyed driving either one. So all i can say is it is a matter of preference.
They are both good cars and Firebirds didnt go out of production because of Mustangs.
The Firebird & Camaro mainly went out of production due to lack of interest. They were too expensive and the dealers never bargained on them when new. A 6 banger Firebird and Camaro were more expensive than the V8 Mustang. They did have some quality issues in the late 80s to mid 90s which they never really got over plus the car remained somewhat unchanged. They pretty much did themselves in.
I don’t care. Since Pontiac stopped making V8’s in 1981 Firebird’s after that had Chevy V8’s.