^^OOOOOO, okay, sorry, misunderstood.
Well, the sound between the Mockingbird and the Firebird won't be drastically different. There's more than that, the other main factor is feel of the guitar. For all I know, Slash picked up that Firebird and loved the feel of the neck and the playability far more than that of the Mockingbird.
Seeing as he's got more workings with Gibson guitars right now, I'd say that COULD have a factor in the switch.