Many G.I. Joe and Cobra vehicles appear in the cartoon when they were still in the development phase. The Firebat is no exception. Initially intended to be sold on its own, Firebats appeared late in the 1st full season of the G.I. Joe: A Real American Hero series, in their pre-production gray color.
The most noteworthy appearance of the early Firebat was in Lights! Camera! Cobra!, where a captured prototype was used along with other authentic vehicles for a G.I. Joe movie (foreshadowing?). After a reminder from Destro that he still owed $15 Million for its development, Cobra Commander orders his troops to recover the confiscated plane.
(Ironically, the Firebat's pilots -- A.V.A.Cs -- appear in another 1st season episode, Worlds Without End flying Rattlers in an alternate dimension.)
The correctly colored red Firebats do not show up until the 2nd season, along with the Terrordrome base. In most of its scenes, it is still flown by Cobra Troopers (and occasionally Wild Weasel). The opening of G.I. Joe: The Movie shows one of few scenes with a Firebat piloted by an A.V.A.C.