The Reds dominated much of the game, but found themselves heading towards a draw thanks to Kylian Mbappe's leveller seven minutes from time before.
Firmino, who faced a late fitness check following an eye injury sustained in the match against Tottenham Hotspurs at Wembley last weekend, provided the match winner as he created an opportunity for himself and stole the headlines a minute into injury time.
All Liverpool First-Half
The hosts dominated the vast majority of the first-half, penning the French champions back with an array of corners early in the game.
Though the majority of those corners came to nothing, Andy Robertson eventually created the chance that lead to the break through.
Trent Alexander-Arnold whipped in the initial ball, missing Sadio Mane at the back post. Scotland captain Robertson retrieved the ball on the left-hand side of the area, whipping in a fierce ball between Thiago Silva and Presnel Kimpebe.
Daniel Sturridge rose highest, meeting the cross with a fierce header that almost went through PSG goalkeeper Alphonse Areola to give Liverpool the lead on the half-hour mark.
With the wind in their sails, Liverpool continued to press PSG in search for a second.
The ever-reliable James Milner stepped up, and converted with an efficient effort into the bottom right-hand corner to double the scoreline.
Only a few minutes later Liverpool found themselves only a goal to the good once again however, as Thomas Meunier found the net with a beautifully struck half-volley on his weaker left side as the ball cannoned off of Robertson in the area.
Mbappe Shines, But Firmino The Hero
With all to play for in the second-half, the two teams took to the field knowing a victory could send them top of Group C in the early stages.
Chances came almost few and far between for much of the second-half, but teenage French sensation Mbappe didn't waste his opportunity when the ball fell to him in the 83rd minute.
With the ball breaking loose on the edge of the box, he drove home a fierce effort into the bottom right-hand corner to level the scores, past the outstretched Alisson.
The Reds pushed hard to get back on top with ten minutes remaining, and substitute Roberto Firmino would eventually be the difference maker.
Picking up the ball on the right-hand side following a deep corner, Firmino feigned on his right only to cut inside on his left, driving home a low effort across the PSG goal and into the bottom left-hand corner of Areola's net, giving Liverpool all three points at the death.
Liverpool will now head into their next fixture against Southampton at Anfield having won six games out of six, joint top of the Premier League and top of Champions League Group C.