Born in Rome, Firmani began his career with local A.S. Lodigiani in 1995, moving the following season to Vicenza Calcio. He remained at the latter club for seven years, making his Serie A debut on 26 January 1997 in a 3–2 home win against ACF Fiorentina and going on to collect nearly 100 official appearances. He also scored in the quarter-finals of the 1997–98 UEFA Cup Winners' Cup against Roda JC, in a 9–1 aggregate routing. During his time in Veneto, Firmani headed south for a year, spending the 1998–99 campaignon loan at Reggina Calcio and splitting 2001–02 between A.C. Chievo Verona and Bologna F.C. 1909. After competing in Serie B in 2002–03 with S.S.C. Venezia, he was finally released and joined Calcio Catania, helping them to two mid-table positions in the competition. The following year, Firmani signed with hometown's S.S. Lazio, replacing Juventus-bound Giuliano Giannichedda. He made a good start to his career in the capital, but suffered a serious injury in November 2005, causing him to miss to rest of the campaign. On 25 November 2007, Firmani scored his first goal for the Biancocelesti against Parma F.C. at the Stadio Olimpico, in a 1–0 victory. The goal, which was scored in the 90th minute, came only a week after the death of his good friend Gabriele Sandri, who was shot by a policeman, and the player's celebration with fans in Curva Nord earnt him their recognition; due to injuries, he only managed seven appearances for the season but still netted three times, the other two being successful strikes against U.S. Città di Palermo and S.S.C. Napoli. Firmani's streak with injuries continued throughout the following campaign and, due to several physical problems, he did not feature whatsoever for his team. He did however, receive a red card during the 1–4 home loss to Cagliari Calcio, when he reacted to an incident involving opponent Daniele Conti and Goran Pandev. He received adulation and support from the Lazio fans following the incident, due to the insipid nature of the team's performance – a popular message was then adopted by the side's faithful, Noi vogliamo undici Firmani. Shortly after the incident, Firmani was requested by Lazio to accept a loan move that would help him regain match fitness. The club suggested some Saudi teams, but the player instead requested he joined Al-Wasl F.C. in the United Arab Emirates; subsequently, the Italians accepted the latter offer. Firmani returned to Lazio in July 2009, starting preseason training in the northern town of Auronzo di Cadore and trying to earn himself a place in new coach Davide Ballardini's team. In February 2011, after being released upon his request, the 33-year-old joined Chinese Super League side Shaanxi Renhe FC.
