I used the firefly a few times before I quit playing, I honestly never gave it much of a chance because of a few things:
A) Its top speed is 22mph, a Tiger is 25mph, Panther is 29mph and T-34 is 32mph
It makes a huge difference when you are trying to gun and run when you are slightly slower then every opponent you will face.
B) Its armor - you CANNOT take a hit from any other tank period, two deaths I had were at 1500 yards from Panzer H's.
C) Its perk Value is 5-9 usually, you are getting an extremely GOOD gun with mediocre armor, the perk is quite good for it.
Your main asset is the 77mm gun, I had a shootout few tours ago at 1600 yards and blew the turret off a Panther, I died to Panzer of course from the same range - that speaks for the armor - long as you score the first hits you can kill a Panther/Tiger at a pretty decent range.
Considering the perks, i think Its a pretty good deal, long as you manage to kill a few tanks you can break even where a Panther I've killed up to 40-50 tanks and still lost perks.
However consider how many perks I have in vehicles, I rather drive a panther for the speed and armor upgrade and its only 10 perks more usually.
Edited:
I have honestly nothing bad to say about the Firefly, being its very low perked - its gun makes up for what it loses otherwise - its a 1 shot knock out tank most of the time, for the perks its right.