The Brazilian striker struck from the far post, completing a rousing comeback for the Merseyside club after Arsenal took a two-goal lead at Anfield.
Gabriel Martinelli and Gabriel Jesus put the league leaders ahead. But Mohamed Salah cut the gap before halftime and missed a penalty after the break.
As Arsenal looked to increase their lead over Manchester City to eight points, Firmino scored what could be the decisive goal in the race for the title.
Things were going well for the visitors as Martinelli put them ahead in the eighth minute, taking advantage of a Virgil van Dijk error.
The forward burst towards goal after Van Dijk fell straight into his path when attempting a shot, and shot low past Alisson.
Arsenal took a 2-0 lead in the 28th minute thanks to Gabriel Jesus’ goal, and Martinelli was the provider, passing the ball to his Brazilian, who shot from close range.
Mikel Arteta’s team went forward, and the crowd at Anfield was subdued until Liverpool responded.
Salah scored in the 42nd minute after Jordan Henderson’s inaccurate shot on goal.
Liverpool went ahead in the second half and had a chance to equalize when a penalty was awarded seven minutes into the match.
Rob Holding brought down Diogo Jota, but Salah was unable to capitalize on the momentum, shooting past.
Liverpool had more chances and substitute Darwin Nunez forced a save from Aaron Ramsdale, but before Firmino’s goal Arsenal looked like holding on for a crucial win.
Things could have been even worse for the London club had it not been for two saves by Ramsdale in the closing stages when Salah and Ibrahima Konate failed to score.
Meanwhile, the organization in charge of professional referees said it would review the clash between Andy Robertson and assistant referee Konstantin Hatzidakis.
The video replay showed Hatzidakis raising his hand when he collided with the Liverpool defender.