With the game deadlocked at 0-0 during the first half, both Mane and Firmino chased a lofted ball over the top from Trent Alexander-Arnold.
Firmino arrived first and chipped over the head of Bernd Leno, with the ball landing neatly for his strike partner after hitting the post on the Brazilian's original effort.
Liverpool's joy quickly turned to bewilderment, however, as the goal was rubbed out for an alleged offside.
Mane was indeed past the last defender in the original move, but did not interfere with play; replays of Firmino's shot, meanwhile, showed the Senegal international dead level with his team-mate.
No matter what the intricacies of the offside law, one thing is for certain: the call went down very badly on the red side of Merseyside - though were a few north Londoners who were happy with the call!
It's a debatable offside. Mane was behind Firmino when he lobbed the keeper, but offside as the ball was played through.
2nd phase Mane was onside, so the goal should have been given. — This Is Anfield (@thisisanfield) 3 de noviembre de 2018
Looks like that was a perfectly goal, depending on when you judge a player being active. Mane definitely not offside from Firmino shot that hit woodwork. — Ian Doyle (@IanDoyleSport) 3 de noviembre de 2018
Arsenal got away with one there. Clearly not offside. — Gary Lineker (@GaryLineker) 3 de noviembre de 2018
How are the commentators AND an ex ref saying Mane was onside and the goal was good? 🤦🏻♂️ the rule is 2nd to last defender not the last defender! He is offside. These people are getting paid to commentate and don’t even know the rules 💩💩💩 pic.twitter.com/jQBDUf35cd — Sean Hansen (@Sean_Hansen) 3 de noviembre de 2018
Lucky Arsenal. No way Mane is offside there— Wark Woldbridge (@markgoldbridge) 3 de noviembre de 2018
Another HUGE error for referees tonight... Sadio Mane's goal wrongfully disallowed for offside. Such a shame that the standard of English referees continues to blight the Premier League. Everyone deserves better. #LFC #ARSLIV #EPL #Emirates #Liverpool #Arsenal #SimonLong pic.twitter.com/Nyb2q3guji — LFC Photo (@LFCphoto) 3 de noviembre de 2018
Shouldn't that have been a goal? Mane offside from the TAA pass but not from the Firmino touch? — Jack Pitt-Brooke (@JackPittBrooke) 3 de noviembre de 2018