Firmino showed some signs of why some are excited about him. He's very aware, his first touch is impeccable, and he's pretty aggressive. That said I'm surprised you thought he was brilliant.
He also conceded possession too much by dawdling on the dribble, or by over elaborating. He was also invisible when Arsenal were dominating (may have just tired), and he left Clyne alone in some dangerous situations. If he was even more productive, and he easily could be as he gains confidence and an understanding of the team, then his lack of impact off the ball will be far less important. If that doesn't happen, he needs more discipline.