The Brazilian, who suffered an eye injury during Saturday's Premier League win at Tottenham Hotspur, came off the bench on 70 minutes to make his 150th appearance for the Reds and hit the decisive strike to move up to joint-third in the club's European Cup goalscorers list.
Firmino bamboozled three PSG defenders and fired into the far corner to notch his 12th for Liverpool in UEFA's flagship competition.
The 26-year-old thereby moved level with Dirk Kuyt and Terry McDermott, with only Ian Rush (14) and Steven Gerrard (30) ahead of him in the Reds' all-time standings.
With a minimum of five games still to play in Group C this season - home and away against Napoli and Red Star Belgrade, plus the return fixture against PSG - it's likely that Firmino will catch Liverpool legend Rush.
The forward has scored his 12 goals in 16 matches and at his current strike rate of 0.75 per game, Firmino would require another 24 Champions League outings to catch Kop icon Gerrard.