Jurgen Klopp's team has already beaten the Estadio do Dragao twice in recent seasons, winning 5-0 in 2017/18 and 4-1 the following year, though the Liverpool manager dismissed the results as having any effect on Tuesday's trip.
But it was much the same for Liverpool, who put up a devastating performance. Salah (18) and Sadio Mane (45) scored tap-in goals in the first half as the visitors dominated the game.
Following the break, they maintained control of the game and increased their lead when Salah (60) tucked home his second goal of the evening after more outstanding work from Curtis Jones.
After a lengthy VAR check for offside, Firmino's second goal came four minutes later, with the striker deemed to be onside as he swept home in the 81st minute.
HOW THE MATCH WENT DOWN:
In the UEFA Champions League, Liverpool has been Porto's bogey team, with the Merseyside being undefeated in eight meetings, and since Jurgen Klopp's reign began, the 'Reds' have won three of their four matches, scoring eleven goals. The hosts started brilliantly in this match, but despite their efforts, they soon found themselves down to an unavoidable Salah goal. Curtis Jones found himself in shooting range again after his goal against Brentford, and Diogo Costa could only parry towards Liverpool's Egyptian winger, who easily tapped into an open net.
At the Estadio do Dragao, the ‘Reds' were in cruise control, and a second goal was on the way. On his 400th appearance for Liverpool, Jordan Henderson was denied, but Mane was on hand for his typical goal against Porto, similarly tapping in from close range. The Senegalese's goal was his fifth against the "Dragons," making him the UCL's all-time leading scorer against a single opponent.
The six-time European winners were simply too strong once more, as Mohamed Salah added a third goal after Jones's exquisite build-up play. Porto had a chance when Mehdi Taremi headed in to make it 3-1, but all hopes were dashed when Roberto Firmino came off the substitute to score two late goals for himself. The Brazilian took advantage of Costa's ridiculous error of being well off his goal line before pouncing on a deflected Jones shot in the box and finished with ease.
Liverpool now have a comfortable six-point lead in Group B, and their unbeaten run in all competitions now stands at eighteen games.
Porto: Costa (5), Corona (5), Cardoso (5), Marcano (6), Zaidu (6), Uribe (5), Oliveira (5), Otavio (1), Martinez (5), Diaz (7), Taremi (5).
Subs used: Vieira (6), Grujic (5), Wendell (5), Vitinha (6), Pepe (5).
Liverpool: Alisson (6), Milner (7), Matip (7), Van Dijk (7), Robertson (8), Fabinho (7), Jones (9), Henderson (8), Mane (8), Salah (8), Jota (7).
Subs used: Gomez (6), Firmino (7), Minamino (6), Oxlade-Chamberlain (6), Origi (1).
Man Of The Match: Curtis Jones.
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