Born in Sussex into a Royal Navy family, Firth was a chorister at Winchester Cathedral before going to Marlborough College[8] and then to the University of Bristol[9] where he secured a First Class Honours degree in Civil Engineering in 1979.
[citation needed] Firth joined specialist consulting engineers Flint & Neill[9] in London upon graduation, and initially worked on a variety of projects including tall radio masts and towers, chimneys and steel buildings, as well as the aerodynamic stability of box girder bridges.
He then attended Imperial College[citation needed] and obtained a further degree in Structural steel Design in 1982.
[7] He became partner in Flint & Neill[9] in 1990, and when the firm joined COWI in 2008 he was appointed a director.
[13] He retired from full-time employment in 2018 and now works as a consultant, continuing to provide bridge design and engineering advice, training and education.