Firmino opened the scoring for Brazil after latching on to a pass from Manchester City forward Gabriel Jesus in the ninth minute.
But Mane got Senegal back into the game after drifting past Dani Alves and Marquinhos and being taken down by the latter in the box.
Bristol City’s Famara Diedhiou stepped up and slotted the penalty past Manchester City goalkeeper Ederson in first-half injury-time.
Neymar, who won his 100th cap for Brazil, could have won it late on for his country, but saw his effort saved by Senegal stopper Alfred Gomis.
A draw appeared to be a fair result with both sides having four shots on target each.