The pair link up at both club and international level, spending the last two years together at Anfield following Firmino's arrival from Hoffenheim.
Ahead of the Selecao's meeting with England at Wembley Stadium on Tuesday evening, Firmino has talked up his understanding with Coutinho in the current system adopted by Brazil and Liverpool.
"Coutinho is a friend and also a great player," he told The Independent. "He's got incredible passing ability; he knows how to put attackers in on goal.
"I think the Brazil system is similar to the one we use at Liverpool, with lots of movement, players open out wide and quality midfielders who join the attack and score goals.
"It helps having Coutinho with me. He's a 'differential' who helps swing games. The fact we're together day to day at Liverpool means we have a good understanding, so we can use that in Brazil games."
Firmino has played a direct part in nine goals in 15 Liverpool appearances this term, while for Coutinho it is seven in nine.