Firebrace was commissioned into the Royal Artillery in 1908. He was promoted Lieutenant-Colonel in 1936, Colonel in 1937, and retired as a Brigadier in 1946. He was the British military attaché in Riga before the beginning of the Second World War and later in Moscow until 1940 as Head of the British Military Mission in Moscow. He acted as an observer and interpreter for Winston Churchill at the Potsdam and Yalta conferences and when Molotov visited London in 1942 and ran the War Office Russian Liaison Group.