Firth became a member of the Labor Party at the age of 15. She studied at
North Sydney Girls High between 1986 and 1991, before studying Arts/Law at the
University of Sydney between 1992 and 1998. While at university, she was active in student politics. After graduating, she worked as a political staffer, prior to working as an articled clerk at Slater & Gordon in 2001; she then worked as a campaign organiser for the Australian Labor Party (2001–2004). Between 2004 and 2007, she practised as a solicitor with
Slater & Gordon, specialising in asbestos litigation and industrial law. Firth was elected as a
councillor of the
City of Sydney in 2004. During her tenure on Council, she served for a period as Deputy Lord Mayor. Prior to entering state politics, she served on the board of the
Law and Justice Foundation and Aidwatch. Firth was elected to the new seat of Balmain on 24 March 2007. Though there was a swing away from the Labor, she won the seat with a majority of just below 4%. She was sworn in on 2 April 2007, receiving simultaneous appointment as Minister for Women, Minister for Science and Medical Research, Minister Assisting the Minister for Health (Cancer) and Minister Assisting the Minister for the Environment, Climate Change and Water (Environment). On 22 February 2008,
Phil Koperberg, Minister for the Environment, Climate Change and Water, resigned from cabinet due to ill health. Firth was subsequently appointed as Minister for Environment and Climate Change. Following the caucus election of
Nathan Rees as Leader of the
NSW Labor Party and Premier in September 2008 and ensuing New South Wales government reshuffle, Firth was appointed as Minister for Education and Training. She won early praise for her demand to the federal government that it fund state public schools to the same level as private schools. During her tenure as Education Minister, she was regarded as a future Labor leader. Firth retained the Education portfolio under Rees' successor,
Kristina Keneally. During early 2009, the then Rees Labor Government announced the
CBD Metro project. Controversial from the start, the CBD Metro project dominated the Sydney news for much of the year. Firth received media coverage for her opposition to the CBD metro, in the event that it would mean a bus interchange at
Rozelle (due to the
urban density of the area). Firth was also perceived as being at odds with economically conservative sections of the NSW Cabinet after her opposition to the privatisation of
Sydney Ferries became public; at the time, the option of privatising Sydney's ferry fleet was examined by the NSW government against the opposition of unions and some residents. The Keneally government subsequently announced that the ferries would not be privatised. Firth was defeated by
Jamie Parker (representing the
NSW Greens) as the Member for Balmain at the 2011 state election. After her defeat, Firth considered returning to work as a solicitor. Instead, however, she was appointed as CEO of the Public Education Foundation, an advocacy organisation providing scholarships for students in need of financial assistance. In May 2014, Firth again won Labor preselection in the seat of Balmain for the 2015 state election. She lost the election to then sitting member,
Jamie Parker. Firth was appointed a
Member of the Order of Australia in the
2023 Australia Day Honours. ==Personal life==