Reds forward Firmino came off the bench early in the second half of his country's final Copa America warm-up match - and quickly added the sixth goal of a 7-0 win over Honduras with a delightful chip.
Alisson Becker started and finished the resounding friendly victory in Porto Alegre.
Five members of Jürgen Klopp's squad were involved in UEFA Nations League action, meanwhile.
Virgil van Dijk and Georginio Wijnaldum's Netherlands were defeated 1-0 by Portugal in the final, after Trent Alexander-Arnold and Joe Gomez had completed the full 120 minutes as England overcame Switzerland 6-5 on penalites following a goalless draw in the third-place play-off.