Assurances the scratch on the cornea had cleared up only came hours before kickoff in the Champions League on Tuesday.
"I can open the eye again," Firmino told Juergen Klopp, according to his manager after the 3-2 victory.
Firmino still had to bide his time to play, starting on the bench and watching as Liverpool threw away a two-goal lead. Firmino had been on the pitch 11 minutes by the time Kylian Mbappe equalized for the French champions in the 83rd minute.
But the French World Cup winner's carelessness at the other end allowed Firmino to provide a thrilling climax to a pulsating night at Anfield to open Group C.
Mbappe was dispossessed by James Milner and Joe Gomez and Virgil van Dijk combined to feed the ball to Firmino. The Brazil forward, who injured his eye against Tottenham on Saturday, now had the vision to find the space to weave through the defense and strike the ball low into the net.
"The day before yesterday there was no chance (he could play) and yesterday it didn't look well," Klopp said. "This morning he came in and said 'I'm fine.'"
It all made for painful viewing for PSG coach Thomas Tuchel in his first Champions League game with the French champions. Conceding in the first minute of stoppage time at the end of an electrifying encounter at Liverpool is just what happened when Tuchel's Borussia Dortmund was beaten 4-3 by Klopp's side in the Europa League quarterfinals in 2016.
"This is what Liverpool do," Tuchel said. "They press you and they make it hard for you in possession."
Four months after the five-time European champions lost last season's final to Real Madrid, Liverpool showed its credentials to challenge for the title again by finding a way to tame Neymar and Mbappe for much of the game.
"He's got to work hard to try to improve," Tuchel said of Mbappe.
Left back Andy Robertson, who repelled Mbappe's forays down the right, also showed his attacking ability by setting up the opener. A cross from Robertson was headed in by Daniel Sturridge 30 minutes into his first Champions League start since playing for Chelsea in 2012.
"We are good at getting in people's faces," Robertson said, "especially at home to make it uncomfortable."
Milner, who was Liverpool's tireless engine in midfield, then stepped up to the penalty spot when Georginio Wijnaldum was tripped by Juan Bern, doubling the lead in the 36th minute.
"We conceded goals in the first half," Tuchel said, "but we never lost our confidence."
That was evident when Thomas Meunier sliced into the net in the 40th minute, and PSG avoided a bigger deficit when Mohamed Salah's goal in the 67th was ruled out for Sturridge sinking his studs into goalkeeper Alphonse Areola in the buildup.
Salah, Liverpool's top scorer last season, had a low-key game and it was his misplaced pass that gifted PSG a chance to equalize. Substitute Julian Draxler took the ball forward and released Neymar, who was challenged by van Dijk as he broke into the area. Mbappe seized on the loose ball and fired into the bottom corner.
"In the second half I think we showed a bit more personality," PSG defender Thiago Silva said. "But in a match like this, where the level is high, you always have to be focused, because the other team can score at any given moment."
As Firmino showed after Liverpool seized on Mbappe's misfortune to make it six wins out of six in all competitions this season.
"We lost a ball that usually we don't lose," Thiago Silva said, "and we were punished for that mistake."
PSG is rarely so troubled at home, already sitting five points clear by winning the opening five games of its French title defense. The harder task is wining the Champions League for the first time.
"Maybe today we could have displayed a little bit more aggression," Tuchel said. "Maybe we need to improve in our intensity in some areas of the field."