Fitzgibbon subsequently transferred to the Roosters in 2000. He played at second-row forward and scored a try in the Roosters' 2000 NRL Grand Final loss to the Brisbane Broncos. Eventually, Fitzgibbon won a premiership with the Roosters, playing at second-row forward in their 2002 NRL grand final victory over the New Zealand Warriors and winning the Clive Churchill Medal for his best-on-ground performance. Having won the 2002 NRL Premiership, the Roosters travelled to England to play the 2003 World Club Challenge against Super League champions, St Helens R.F.C.. Fitzgibbon played at second-row forward, scoring a try and kicking nine goals in Sydney's victory. In the 2003 NRL grand final Fitzgibbon played in the second row and was the Roosters' goal-kicker in their loss to the Penrith Panthers. After that he was selected to go on the 2003 Kangaroo tour. Fitzgibbon played for the Roosters at second-row forward in their 2004 NRL grand final loss to cross-Sydney rivals, the Bulldogs. Fitzgibbon was selected in the Australian team to go and compete in the end of season 2004 Rugby League Tri-Nations tournament. In the final against Great Britain he played from the interchange bench and kicked two goals in the Kangaroos' 44–4 victory.