Having served as a cadet in the
Officers' Training Corps, Firth was commissioned as a temporary second lieutenant on 29 August 1914. Firth was a temporary
lieutenant of the
Shropshire Light Infantry when he was appointed a
flying officer on 17 April 1917 and transferred to the General List of the
Royal Flying Corps. Having been assigned to
No. 45 Squadron RFC as a
Sopwith 1 1/2 Strutter pilot, he scored his first win on 12 July 1917; he helped drive down a German
Albatros D.III fighter out of control east of
Messines. Ten days later, he would drive another one down over
Menen. The squadron then re-equipped with
Sopwith Camels; Firth would use this craft for his remaining nine victories. He began by driving down an Albatros two-seater reconnaissance plane over
Passchendaele on 25 September 1917. The next day, he destroyed an
Albatros D.III in the same vicinity. He set an Albatros D.III aflame on 26 October, to become an ace. On 31 October, he and
Peter Carpenter shared in the destruction of a German
reconnaissance plane. He scored three "out of control" victories during November to end the year 1917 with nine wins. "He has on various occasions, during a period of two months, completely destroyed two enemy planes and shot down out of control seven others. The latter, by reason of the manner in which they were observed to go to earth, were probably all rendered useless for further service. He has set a very fine example as a patrol leader, and has displayed much skill and courage." He was also awarded the Italian
Bronze Medal of Military Valor on 2 November 1918. ==Post war==