The 24-year-old joined under Brendan Rodgers's era at Anfield.
Despite an indifferent first few months in a red shirt, the 24-year-old has gradually found his feet and he looks like a player who the ex-Borussia Dortmund coach could develop even further in the coming years.
The Brazil international, who Klopp would have known as their paths crossed in Germany, albeit for different clubs, scored his 11th goal of the season in Sunday's 2-0 win over Watford, and the 48-year-old admits he was surprised to hear that the Reds had pulled off such a coup last summer.
“I knew about Roberto. When I heard that Liverpool had signed him I thought: ‘how have they done this?," he said to the Liverpool Echo.
“Now with Roberto you can see the confidence in his work. All the players are more confident than before because they’ve had more time working together.
“We’ve had an intense time but it’s been a good time together. We know a lot more about each other."
One of Firmino's best displays in a Liverpool shirt came at Anfield last Thursday when he helped his side to a 3-0 Europa League win over Villarreal, booking a final spot in the process.
Klopp and co will face reigning champions Sevilla in Basle on Wednesday week.