The modified Sherman known as the Firefly was exclusively used by the British and their respective allies, but the Americans interestingly did make extremely limited use of Shermans that they re-armed with the 17-pounder gun at their own volition very late in the war. In early 1944, the United States tested the 17-pounder gun for use in their own tanks, placing a proper Firefly turret (that is, one modified in Britain to British specifications) onto a wet-stowage M4A3 hull. The gun itself was rejected on the basis of inaccuracy and other factors when compared with the 76 mm M1 and 90 mm M3 guns, and the project died. The Americans experienced heavy tank and armored vehicle losses (>800) against a German force which included a disproportionate number of heavy tanks in the Battle of the Bulge; this prompted three things to happen in early 1945:
The shipment of any more 75 mm-gunned Shermans as replacement tanks to the European Theater of Operations was stopped
Existing 75 mm-gunned tanks were re-armed with 76 mm guns
The program of re-arming American Sherman tanks with the 17-pounder gun which had been aborted some time before was restarted
In mid-March 1945 it was decided to modify 300 75 mm-gunned Shermans to carry the 76 mm gun. Re-armed Shermans featured a modified turret with a large slab counterweight on the rear; the small turret was quite cramped. In light of the end of the war being relatively near, the fact that shipments of new 76 mm-gunned tanks exceeded losses, and the fact that converting substantial numbers of tanks would take too long (45-60 days), the process of re-arming 75 mm-gunned Shermans with 76 mm guns was suspended on April 14, 1945; few examples were produced.
The process of re-arming Shermans with 17-pounder guns did get started, and produced about 100 tanks before the end of the war. M4 and wet-stowage M4A3 tanks were acquired for conversions, with the M4A3 being preferred. Two pilots of M4A3 tanks were completed on March 15, 1945, and the project was cleared to begin; 160 were to be made by April 30, 1945. The tanks featured a distinctly "American" version of the Firefly turret; the radio box on the turret rear was slightly larger to allow the carriage of the SCR-508 or SCR-528 radio, and the machine gun stowage apparatus was mounted on the box; there was no loader's hatch, unlike the British turrets.
Eleven were completed by the end of March, and left England for France. On April 7, 1945 the Army decided that they would only need 80. By the time that news reached those in England doing the conversions, 100 tanks had been completed. The conversions were quickly stopped when the war ended (the last three conversions were completed on May 7, 1945 and left England on May 10) The 20 excess vehicles were later transferred via Lend-Lease to the British Army.
When the tanks trickled into France, the plan was for them to be distributed 40 apiece to units of the First and Ninth US Armies. Some of the vehicles ended up being issued to US units before the war ended. A photo of an M4 (17 pdr) serving with the 3rd Armored Division in France or Belgium exists. It appears that none of the M4A3 (17 pdr) which were converted ended up ever being issued to US units. The fate of the vehicles after the war is unknown; they were rejected for potential use in the Pacific.
Sources:
M4 (76 mm) Sherman Medium Tank 1943-65, by Steven J. Zaloga