Both were involved for the five-time World Cup champions during the recent international break, with Coutinho starting both qualifying matches against Bolivia and Chile.
The midfielder was replaced by Firmino in the closing stages of Wednesday's 3-0 win over Chile, which was enough for Brazil to finish 10 points clear at the top of the South American standings.
Coutinho and Firmino have now returned to Melwood to reconvene with the rest of the squad as Liverpool prepare for the early Saturday kickoff against Manchester United at Anfield.
Coutinho in particular is a doubt for the game due to his schedule during the international break, although he came through both matches unscathed.
Liverpool currently sit seven points adrift of United after just seven games of the new Premier League season.