Having witnessed the massacre of Sherman tanks in North Africa and Southern Europe from the hands of Germany's newest designs, British designers decided that if they cannot up-armour their Lend-Lease Sherman tanks, they'll at least make them able to fire back at German Panthers and Tigers. The gun of their choice was indigenous Ordnance Quick Firing 17 pounder. Interestingly, the gun was rotated by 90* to fit inside the turret. While it was just barely larger in terms of caliber (76,2 mm) than standard Sherman gun (75 or 76 mm), the muzzle velocity of its projectiles was much higher, and the shells were designed for anti-tank role from start, unlike American Sherman's 75s and 76s, which were good for firing anti-personnel and anti-building shells, but not anti-tank ones. That's how Sherman Firefly, one of the best Allied tanks of WWII, was created. Fireflies could fire back at German tanks and cause serious damage to them, sometimes forcing them to retreat. While Sherman Firefly was still pretty vurnerable to German 75 mm and 88 mm guns, Allied tankmen felt much more confident in Fireflies than standard M4s. Polish Army in the West also used these tanks.
This tank is in running condition. It was brought to Poland from Belgium, but I don't know if it was used by Poles during World War II. All I know is that it was used as a target on shooting range and when it was brought to Poland it looked like a cheese grater.
Thanks to the effort of Artur Zys' company, which greatly helped in renovation of such vehicles as StuG IV, SdKfz 6 or BTR-152, it was possible to restore the Firefly to the running order. It'll stay in Poznan until it is moved to Gdansk Museum of Armament in 2014 or so.