Firmino was only fit enough to be on the bench after suffering an eye injury in Liverpool's victory at Tottenham on Saturday - but he emerged on 72 minutes to secure a win that manager Jurgen Klopp may have thought was slipping away after Kylian Mbappe's late equaliser threatened to rescue a point for PSG.Daniel Sturridge, Firmino's replacement in Liverpool's starting line-up, headed them in front from Andrew Robertson's cross on the half hour, James Milner adding a second from the spot six minutes later after Georginio Wijnaldum was fouled.
Thomas Meunier pulled one back for PSG before half-time but, with Brazil superstar Neymar and Mbappe subdued, Liverpool were in control, seeing a Mohamed Salah goal ruled out for a foul.
Klopp's side were stunned when Mbappe pounced in the area to beat Alisson with seven minutes left - but Firmino made it a perfect start to Liverpool's Champions League campaign by beating Alphonse Areola with a low, angled drive deep into stoppage time.
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Reaction from Anfield
Liverpool's relentless spirit wins out
Liverpool could have been forgiven for believing fate was against them when Mbappe made the most of his first serious opportunity to drill a powerful finish low past Alisson in front of the Kop with seven minutes left to draw the visitors level.
Klopp's team had barely been troubled by PSG's star-studded attacking line-up and, if anything, looked more likely to add to their tally than concede.
It was to their credit, and an illustration of the talent and spirit at Klopp's disposal, that Liverpool gathered their strength for one last push that delivered Firmino's winner.Klopp does not just have an array of attacking forces, he has fiercely-driven competitors such as the outstanding James Milner and Andrew Robertson to set the tone and the example for others to follow.
Liverpool proved once more that they will pose a huge threat in this competition because they are capable of scoring goals at any time from any angle from a variety of players - it is a weapon that will make them serious contenders if they can make their way into the knockout phase, as they proved last season.
And this opening victory against the side likely to be their main challengers for top spot in Group C, is the perfect opening.
Liverpool demonstrated their pedigree and danger when reaching last season's Champions League final. This result against another team with designs on the trophy is an early warning sign for the current campaign.Sturridge delivers a timely reminder
This was Sturridge's first start in a Champions League game since March 2012 when Chelsea faced Napoli - 2,379 days ago - and he made up for those lean years as he grabbed this opportunity.
Sturridge is making his latest attempt to rebuild his career after falling into the shadows because of frequent injury problems and the emergence of the attacking trident of Firmino, Sadio Mane and Salah.
It looked like he would not even get the chance as he was out on loan at relegated West Bromwich Albion last season, a spell wrecked almost immediately by a hamstring injury.
Instead, he has returned to Liverpool with a burning desire to get back in contention, perhaps fuelled by the realisation it might never get better for him than it is right now at Anfield.
The 29-year-old started for Liverpool because of the eye injury Firmino sustained at Spurs and the warm embrace he received from Klopp when he was replaced by the Brazilian with 19 minutes left would have been appreciated.
Daniel Sturridge celebrates scoring for Liverpool against Paris St-Germain in the Champions League
Daniel Sturridge has been directly involved in a goal in each of his last four Champions League appearances for Liverpool, claiming two goals and two assists
Sturridge may not be the razor-sharp operator of old but he can still play a part and his goal, headed in from Robertson's perfect cross, was a reminder that there may yet be something that a rare natural talent can still offer.
He wasted a good opportunity in the second half with a poor header but this was the sort of performance - and the sort of stage - Sturridge needs to prove himself once more.
Tame PSG fail to fire
Paris St-Germain may have thought a timid, cautious performance might have gained them an unlikely point when Mbappe made it 2-2 so late on - but in reality it would have been a reward they barely deserved.
For a side containing superstars such as Mbappe and Neymar, as well as Angel di Maria and Edinson Cavani, it was almost beyond comprehension that they were so short of attacking ideas and variety.
Mbappe buried his one opportunity and Neymar forced a routine save from Alisson in the first half but this was not a game were Liverpool's defence, well-organised as it was, came under any sustained pressure or stress.And it was clearly a matter of huge frustration to PSG's German coach Thomas Tuchel - no stranger to Anfield misery after losing here with Borussia Dortmund in the 2016 Europa League quarter-finals.
He raged in frustration at a half-hearted second-half challenge from Cavani and was equally angered when Neymar made no attempt to regain possession after losing the ball as Liverpool chased the winner in the closing moments.
PSG is a team and club that has been built to win the Champions League. They will have to do a whole lot better than this to achieve their goal.
Man of the match - Georginio Wijnaldum (Liverpool)