In 1943 the British Ministry of Supply rejected the idea of fitting the 17-pounder gun into the Shermans equipping their armored divisions; the new Cromwell tank was initially expected to have a 75mm gun superior to the Sherman's in armor-piercing and high-explosive capacity and a Challenger variant was in the works armed with the 17-pounder. However, Major George Brighty and Lt. Colonel George Witheridge of the Royal Tank Regiment believed the Sherman chassis would be a better mount for the 17-pounder than the Cromwell, and started an unofficial project to build the conversion. Fortunately for the British their efforts turned the Firefly into an official Ministry of Supply project; the Cromwell's turret ring proved too small for the proposed high-powered 75mm gun and the A30 Challenger project proved to be complicated to manufacture and was incapable of being landed across a beach, making it unable to participate in Normandy until the completion of "Mulberry" harbors and the capture of German-held French ports. Only 200 Challengers were produced, as opposed to about 2,000 Fireflies.
Fitting the 17-pounder into the Sherman turret required removing the hull machine gun to fit more ammunition and left no space for a commander's radio set. In addition, the 17-pounder had inferior high-explosive shells to the 75mm guns on the standard Sherman and Cromwell tanks. As a result Fireflies were usually put into combat alongside Shermans or Cromwells, hanging back in ambush positions while British tank commanders led their units in advances. The fearsome performance of the 17-pounder - which had more armor-penetration capability than even the vaunted Kwk 36 and 42 guns of the Tiger I and Panther - made the Fireflies priority targets for German tank crews. Nevertheless, Firefly losses were lighter than those of standard Shermans; while countershading paint on the long gun barrels to make them appear identical to the 75mm was of dubious effect the Firefly's usual tactic of hanging back under cover prevented them from being spotted until they had a chance to strike. Numerous engagements against advanced German tanks proved the Firefly's worth, with some Fireflies obtaining multiple Tiger and Panther kills in a single engagement. Famed SS Tiger ace Michael Wittmann is believed to have been killed in a Firefly ambush in August 1944, and the 17-pounder was the only gun in the Western Allies' arsenal theoretically capable of punching through the Tiger II's heavy frontal armor.
The Firefly is the most dangerous tank vs. tank opponent in the British arsenal, capable of defeating even the heavy German tanks. However, it has no heavier armor than the standard Sherman or Cromwell, and should be used cautiously; if spotted the Tiger I and II can outrange and destroy the Firefly. However, when firing from ambush positions they can lay waste to your German opponent's vaunted panzers.