The Firefly carried no MG and very little HE ammo, making it useless as an infantry support tank. Its role was exclusively that of tank hunter, hiding behind cover and waiting for the big cats to show themselves, then picking them off at long range.
IMO, the criteria should be simple: anything that was bred as a dedicated tank hunter should be allowed. I don't think it would be fair to exclude the Firefly, since it carries a dedicated anti-tank gun and was the king of the allied TD's. It's essentially a slower, heavier armored version of the Achilles, because it carries the exact same armament - it just has a top on its turret.
Something that totes a big gun but was designed as a main battle tank or assualt gun would not qualify - like some of the StuG and SU series. So only the StuGs with the StuK 42 75mm would qualify, and while the SU-76 , SU-85 and SU-100 are TD's, the SU-122 is not.
In the case of a multi-purpose vehicle like the ISU-152, I'm not sure. It was designed as an assualt gun, but soon became known as "beast killer" becuase of its effectiveness against the German heavy tanks. I'd probably include it as well.
Here is a list:
German
StuG III Ausf F, G w/75mm StuK 42
StuG IV
Marder series
SdKfz 251/22
SdKfz 250/10
Jagdpanzer 38(t) Hetzer
Jadpanzer IV series
E-10/E-20
Hornisse/Nashorn
Ferdinand/Elefant
Jagdpanzer V Jagdpanther
Jagdpanzer VI Jagdtiger
US
M3 GMC
M6 Fargo GMC
M10 Wolverine GMC
M18 Hellcat GMC
M36 Jackson GMC
Russia
SU-76
SU-85
SU-100
ISU-122
ISU-152
Great Britan
Firefly
Archer
Achilles
Cruiser Mk VIII Challenger
Some examples of inelligible vehicles would be:
Tiger I & II
Panther
Panzer IV (even 75mm H models)
Early StuG III's with short guns
Sherman M4A3(76)
Super Pershing
SU-152
Comet
Just my thoughts...