Firth was a lawyer who after working in Cyprus for a brief period left to join the Service des Antiquités in Egypt, an organisation run by French scholars to prevent the illicit trade in Egyptian artifacts. Firth worked on the first archaeological survey of Nubia of 1907 to 1911. In 1912 he set up the Aswan Museum. He became the inspector of antiquities at Saqqara and began to explore the complex of Djoser's Step Pyramid in Saqqara where, in 1924, he discovered the serdab of the pharaoh, which is now in the Egyptian Museum in Cairo.
In 1928 and 1929, Firth began the opening of the funerary complex of Userkaf, the first king of the 5th dynasty, and that of a small pyramid located just at the south, allotted to his queen. In early 1930 he worked on the Headless Pyramid, believing it to be a pyramid of a king of the First Intermediate Period. It is now understood that the pyramid belongs to Menkauhor Kaiu of the Fifth Dynasty.