The offence occured on February 4 when a night out with friends resulted in Firdaus consuming alcohol and being involved in a police investigation, although the player denies committing any criminal offence.
However, the FAS has a strong stance on players' off-field behaviour, and the 24-year-old admitted to violating two clauses of the FAS Code of Conduct and Professional Ethics for LIONSXII Players.
Clause 1(b) stipulates that all players are ambassadors of the FAS and must avoid putting themselves in situations that could disparage the image of Singapore, FAS, his club or fellow LIONSXII players, coaches or officials, while Clause 8(f) states "no alcohol shall be consumed at anytime".
The panel that passed the punishment revealed several mitigating factors were considered, including the fact that Firdaus had made a sincere and unqualified apology, had a clean disciplinary record, and was given a vote of confidence from LIONSXII coach V. Sundramoorthy.
Nonetheless, panel member Lim Kia Tong, who is also the chairman of the AFC disciplinary committee, was keen to stress the severity of Firdaus' offence.
"The panel is extremely mindful that the consumption of alcohol by Firdaus to the extent that he required assistance to the toilet to vomit was a conduct which brings negative publicity, embarassment or disrepute to FAS and has tarnished and disparaged the image of Singapore and FAS," Lim said.
Firdaus also expressed deep regret over the incident, and has now set his sights on regaining his place in Sundramoorthy's side.
"I would, once again, like to apologise sincerely to everyone, including FAS, the coaches and players of the LIONSXII, my fans and family for behaving unprofessionally and I deeply regret my actions," Firdaus said.
"The panel was very strict and I was very fearful of the panel terminating my contract. I am grateful that they finally decided to give me a second chance to realise my childhood dream, which is to play for the national team and to continue to play professional football in Singapore.
"This is a heavy punishment, which is nearly three months of my salary. However, I have accepted it. I treasure this chance and I shall constantly remind myself and the other players of the terms of the Code of Conduct which we had promised to comply when we signed it. I shall be a model professional going forward.
"My immediate priority now is to resume training and earn back a place in the LIONSXII team."
Despite this incident, the LIONSXII are doing well on the pitch, and are currently fourth in the Malaysia Super League after Tuesday's 2-1 win over Perak.