The Firefly might technically be an upgunned Sherman, but it's also without its MG and HE ammo meaning it can't be used as an infantry support tank. If you know anything about it (I don't mean that sarcastically) you would find that it was designed and used exclusively as a tank hunter, and treated completely differently in British armor tactics than the standard gun tanks.
It was used as a tank destroyer far more than the StuG III ar IV, which both filled the infantry support role quite often.Admittedly, something like the Panzer IV H wouldn't qualify, because it is more of a main battle tank, or the British Comet, because it too is more of a heavy tank. But if you disqualify the Firefly, to be fair you'll have to disqualify the StuG's, ISU-152, and the Mk VIII Challenger.
The StuG III F and G are just as much upgunned D's as the Firefly is an upgunned M4A4. At least the StuGs still carry half HE ammo and an MG, which is more than the Firefly does.
I wouldn't be making such a stink about any old vehicle, except that the Firefly is the epitome of "tank hunter", at least for the allies. Stuff like the Tiger, obviously not - just because it has a big gun doesn't make it a tank hunter. But I really don't see how the Firefly can be regarded as anything else...it's quite obviously not a main battle tank.
I'm starting to get the idea that fixed casemate = tank destroyer, traversing turret ≠ tank destroyer....