Firth served as a member of the New South Wales Legislative Assembly representing the electorate of Balmain for the Labor Party from 2007[2] to 2011.

[3] On 22 February 2008, Phil Koperberg, Minister for the Environment, Climate Change and Water, resigned from cabinet due to ill health.

[4] 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.

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).

[6] 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.

[citation needed] Firth was defeated by Jamie Parker (representing the NSW Greens) as the Member for Balmain at the 2011 state election.

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 January 2011 Firth's husband, Matthew Chesher, was charged by police for possession of the illegal drug, ecstasy.