Montgomery's 21st Army Group had 342 Sherman Fireflies for the D-Day landings. The British 2nd Army which included the XXX Corps and 2 other Corps was part of the 21st Army Group. Firefly's were distributed in a ratio of 1 Firefly and 3 standard Shermans among the armoured divisions. XX Corps would have had Firefly's when they landed in Normandy.
Are you using it in a Market Garden display? I have seen photos of the Irish Guards Firefly's (part of XXX Corps) during Market Garden in September 1944.
Terry