Firefly is a tier VI British medium tank with the highest standard and premium ammo penetration. 171mm on tier VI sounds great, right? On medium tank platform even better? Nope. It's British.
Brits were bad at flanking (royal noobs) so they added 17-pounder gun to Sherman tank. Who needs maneuver when you may simply penetrate German HTs from front?
17-pounder gun. Do not even bother playing with another gun. You are here for gud pen, gud alpha