Mignolet spent four years with Firmino at Anfield before moving the Belgium in the summer for £6.4million, having signed a five-year contract.
Mignolet fancies Club Brugge duo Hans Vanaken and Ruud Vormer more in front of goal than Firmino - and compared them to super goalscorer Mohamed Salah.
"Both Hans and Ruud are very good finishers," Mignolet told Het Laaste Nieuws.
"Firmino and Mo [Salah] are of course incredible football players, but Firmino is not a born killer.
"Hans and Ruud should certainly not be inferior to Bobby in terms of finishing.
"I even dare to say that Hans and Ruud on that are better than Roberto Firmino. And that they are of a comparable level to Mo."