What exactly makes you think that the firefly will beat a panther in the first place? The firefly had 12 second reload in the alpha, with 240 damage. It has lower penetration than the jackson, despite the much, much longer reload. It's turret rotation was ridiciluosly slow, about as fast as the KTs. Unless it uses it's 150 muni rocket strike, a panther easily beats a firefly. I'm sure the firefly will be buffed though, at those stats it was completely useless.
Now the comet. 185 fuel, lower armor and penetration than the panther, albeit only slightly, good againts infantry, 50 range (same as panther), faster rate of fire.
Anyways, in the alpha, brits were at a significant disadvantage againts ostheer in a 1v1 scenario. Brad developed 1 effective strategy that worked, however it was due to a unit that was heavily overperforming.
In teamgames brits will be a fun faction to play, and balance there doesn't matter because any kind of balance is not possible to achieve there.
In 2v2 brits will be fine.
what makes you think 3vs3 and 4vs4 cant be balanced?