The Liverpool forward converted from close range to give his country an 11th-minute lead, having started the match alongside Reds teammates Alisson Becker and Fabinho.
Neymar added a second from the penalty spot before half-time after Fabinho, who played at right-back, went down inside the USA area.
Alisson and Fabinho completed the match, while Firmino was withdrawn 15 minutes from time.
Brazil are back in action on Tuesday night with another friendly, against El Salvador in Washington.