The main British contribution to the development of Sherman began with the installation of the 76-mm gun. It was known as Sherman Firefly and was the most armed British tank during the war. The installation of the 76-mm gun on the M4 was offered in January 1943 as a guarantee in case of failure of the tank program Challenger, on tests that revealed several shortcomings. Despite some opposition to this decision by the Ministry of Supply, the military decided to build a prototype. In November 1943 it was presented to the customer, and in February 1944 due to delays in the project Challenger and the uncertainty of its future, the alteration of M4 in Firefly got priority, which allowed the troops to get it during the Normandy landings.

Sherman Firefly was the only British tank capable of relatively fight on equal terms with the German Tiger and Panther tanks. Originally Sherman Firefly was introduced at the rate of one tank to the unit due to a shortage of 76-mm tank guns. However, by the beginning of 1945 the tanks of this modification entered the army in increasing quantities. At the end of 1945, the turret of Sherman Firefly was sent to the Aberdeen Proving Ground to be installed on the M4A3 chassis. But the U.S. Army did not adopt it. In the UK, on the chassis Sherman were produced a number of successful emergency vehicles.