IMO, it is easy to see what the Firefly has been left behind. The main culprit is the M4/76. There are couple of very distinct reasons-
In no particular order:
The Firefly is the slowest tank/TD in the game at 20mph. [M4/76 is 26mph]
The Firefly offers nothing the M4/76 cant do inside typical AH tank combat ranges (less than 1600 yards).
The Firefly has a perk price of 7, while the M4/76 has a cost of 2.
The Firefly is a bit slow on the reload at 8 seconds, that is 2 seconds slower than the Panther, 2.5 seconds slower than the M4/76, and 1 second slower than the Tiger. Only the King Tiger (10 sec) and the T34/76 are slower.
It offers no more armor protection (on paper) than the M4/76mm.
So in short, unless the range of the tank battle starts are 2000 yards and only gets further out, there is absolutely no reason to take the 7 point Firefly when the 2 point M4/76 will do everything better than the Firefly except for long range AP. On the same token, why take a Firefly when for a token 8 more perks a faster firing/same powered Panther tank with 9 mph more speed, pivot capability, and vastly superior frontal armor be taken???
I STRONGLY encourage HTC to take a second look at the Firefly, especially the reload rate and the speed. The power of the 17 Pdr gun is not to be under-rated, but the reload rate for the Firefly is simply too slow. I base that on knowing the mechanics and how the main guns works on comparable tank guns (M4/76 and Panther), ammunition storage accessibility on the same tanks, size of the tank rounds in question (Firefly and Panther shells were very near the same weight and length, the M4/76mm was very close as well), and testimony from modern day tankers. HTC's reload rate for the Firefly, as with ALL reload rates of their tanks are purely arbitrary numbers. Regarding the speed of the Firefly, there are multiple sources out there that show that the Firefly was indeed slower than the US M4's for some specific reasons, but none of them gave 20mph as the exact speed. The ranges I've read are 22-23 mph.
edit: sorry for the hijack