And it was the latter pair who combined for the opening goal after 11 minutes: Costa getting the better of Antonee Robinson down the right flank before the Juventus man set up an easy chance for Firmino.
The goal was Neymar's 58th for his country in 91 matches.
Brazil put their World Cup disappointment behind them as the Selecao overpowered a youthful US side 2-0. New general manager Earnie Stewart said this week a permanent coach will be announced later this year.
Just 32,469 attended the match at MetLife Stadium, a possible site of the 2026 World Cup final, and Brazilian supporters vastly outnumbered American fans. When Brazil beat the US 2-0 at Metlife Stadium in August 2010 shortly after the venue opened, the game drew 77,223.
The visitors were on top throughout against a young US team that have been without a full-time coach since Bruce Arena resigned after they missed out on the World Cup finals for the first time since 1986.
Neymar, in turn, said he had accepted the armband "because I've learned a lot and I will learn much more, and this responsibility will be a good thing for me".
DeAndre Yedlin battled well all game against Neymar, using his pace to limit the winger's impact, the Newcastle defender also able to get forward a number of times, especially in the first half, to help create some unsafe moves of their own.
Neymar, who now plays for France's PSG, converted the penalty with a soft shot to the left as Steffen lunged to the right.
However, earlier this week, Neymar insisted that he's making an effort not to be so theatric in the future.
Arthur made his Brazil debut in the 60th minute, Lucas Paqueta in the 70th, Richarlison in the 75th and Everton in the 80th.