Yes not many Sherman Fireflies were ever built, and it was a rather unique vehicle which could justify it being a premium only.
HOWEVER, if the Firefly would be a premium only, then the Jumbo shouldn’t even exist to begin with.
about 2000 Fireflies were ever constructed - while the total amount of Sherman tanks ranged at around 50000 total vehicles.
Now if I tell you there were only ever 250 Jumbo tanks, of which 100 during war time got upgraded with the 76mm gun, you would surely see that the 17 pounder Sherman deserves to be more represented in the game