Fabinho, 24, makes the move to Anfield for a reported £44 million (€50m/$58m), agreeing to a long-term deal.
The versatile midfielder and defender said he spoke to Firmino – who enjoyed a stellar campaign for Liverpool, scoring 27 goals – before opting to make the switch.
"We spoke a bit more about how the city was, about the infrastructure of the club, the city and we also spoke about the coaches and how they do their work," Fabinho told the club's website.
"So I was able to know a bit more about Liverpool through Firmino. I had been with him at the Brazilian national team on a few occasions.
"I know he's a fantastic person, his background is from nice people. He is also very good for someone who comes to a new country, a new city – someone who speaks the same language.
"This will of course make the adaptation much easier."
Liverpool manager Jurgen Klopp spoke of his excitement about Fabinho's versatility after the signing was confirmed.
Fabinho – who helped Monaco win the Ligue 1 title in 2016-17 – hopes he can adapt to the demands of the Premier League.
"I believe I am a player that can organise the game well for my team. I play with a lot of intensity and I'm quite strong in man-marking," he said.
"Unfortunately, occasionally I get some yellow cards for that but I think that won't be a problem here.
"Hopefully I will have no difficulty in adapting to this type of football. I know that the type of football here is quite intense and physical but I think I got some of that while playing in France and I hope I won't find any problems to adapt to this league."