He returned to England in 1963, linking up again with Charlton. After two years he moved to Southend United but returned to The Valley for a third spell with Charlton two seasons later. In total, he made 177 appearances for the Addicks, scoring 89 goals. In recognition of his talents, he was named as Charlton's greatest overseas player in 2005. Firmani is the only man ever to have scored 100 League goals in both England and Italy. In 1975, he played one game for the Tampa Bay Rowdies of the North American Soccer League.
In 1967 Firmani was appointed Charlton manager and retired as a player. Firmani thus became the first Italian international to manage an English club. In his second season in charge, Charlton finished third in the Second Division just missing out on promotion. He was fired in March 1970 with the Addicks battling against relegation to the Third Division.
In 1977 Firmani was signed by the New York Cosmos after having resigned from the Rowdies for personal reasons. Cosmos then had a team featuring Pele, Beckenbauer, Giorgio Chinaglia et al. Firmani added Carlos Alberto, Brazil's World Cup-winning captain of 1970, and the Cosmos won back-to-back titles in 1977 and 1978.
In 1960 Firmani wrote a volume of autobiography "Football with the Millionaires", which provides an interesting contrast between the lifestyle of Italian footballers and their English counterparts in the era of the maximum wage.
Firmani coached in the Middle East during the mid-to-late 1980s, and in the early 1990s. He worked as a manager in both Kuwait and later in Oman. He mentioned in an interview how he and his wife had "liked it in Oman", and then described how he had led Sur to the first division for the first time in the club's history. After Sur was promoted to first division, Mohammed Bahwan, son of famous Omani car-dealership owner, Saud Bahwan, gave Firmani and members of the club large bounuses.