Since he will be leaving from the club with free transfer for the club and there will be lots of offer from another club. The club like barcelona will be also hunting him but it's not yet clear for now. the thing that is clear if al nassr has ever given him an offer.
Barcelona already have Lewandowksi, so if Firmino accept to join Barcelona, he will always become a substitute player or he will play if Lewandowski is injured. But there's no point if he join to Al Nassr since he's a striker, his position and Cristiano Ronaldo is same, except he's agree to play at central attacking midfielder or as a winger. After all he will support Cristiano Ronaldo to score a goal.
The rumor, according to El Nacional via the Madrid-Barcelona digital portal, Firmino has been offered to FC Barcelona through his representative who has contacted President Joan Laporta to explore the possibility of playing at Camp Nou next season. even so, I do not believe in the legitimacy regarding the rumor. because, I did not find it from several other sources. however, it is possible that Firmino will play with Barcelona next season.
Apart from that, I think Serie A clubs are the ideal place for him to play as his new club. because, there he can play as a regular player who has better playing minutes. in fact, Juve, Inter, are interested in bringing him in next summer. however, the club most interested in signing him is AC Milan. So, I doubt if Firmino will join Al Nassr.