As a child, Firth travelled a lot due to his parents' work, spending some years in Nigeria. He also lived in St. Louis, Missouri when he was 11. Firth described this as a difficult time, and he struggled to fit in. On returning to the UK he attended the Montgomery of Alamein Secondary School (now Kings' School), which at the time was a state comprehensive school in Winchester, Hampshire. He was still an outsider and was the target of bullying. To counter this he adopted the local working class Hampshire accent, and affected a disinterest in schoolwork. By the time he was fourteen, Firth had already decided he wanted to be a professional actor, having attended drama workshops from the age of ten. Until further education, he was not academically inclined, later saying in an interview "I didn't like school. I just thought it was boring and mediocre and nothing they taught me seemed to be of any interest at all." However, at Barton Peveril Sixth Form College in Eastleigh he became instilled with a love of English literature thanks to an enthusiastic teacher, Penny Edwards, and has said that "My two years at Barton Peveril were among the two happiest years of my life".
After his sixth form years, Firth moved to London and joined the National Youth Theatre, where he made many contacts in the acting world, from which he got a job in the wardrobe department at the National Theatre. From there he went on to study at Drama Centre London.
