Really? I thought the Firefly was the slowest shooting tank in the game.
Fireflies reload 8-8 seconds.
Panthers reload 5.8-6.4 seconds.
At vet 2 Fireflies drop to 6 seconds to reload.
At vet 3 Panthers drop to about 4-4.5 seconds to reload.
So a vet 2 firefly can reload faster than a subvet3 panther by RNG.
The difference is that Fireflies have 10 more range, way superior accuracy (and on the move) and 40 more damage. They get 80 more damage with vet3. (Vet 3 fireflies would've matched FBP Elefants and JTs for damage per shot.) They certainly don't have the armor and they don't have the health pool of the panther. But against panthers, that weakness is directly mitigated by their damage output.
Panthers get 10% more armor and 160 health at vet 2. Their faster reload is more than mitigated by their terrible accuracy.
A panther's chance to hit is more or less the Firefly's chance to penetrate. But when moving, a panther's chance to hit plummets and can expose its rear armor which a firefly will always penetrate. A moving firefly's chance to hit is much less impacted by moving accuracy and its own raw accuracy, and it's armor is always going to be penetrated by panthers.