I think what OP wanted to communicate is that, in his mind, firefly should not be available as early as cromwell. I don't think this is a valid point though becouse all factions get their best respective non doc TDs roughly at the same time and exatly at the same time as their first medium.
For example:
soviets: T4: su85 and t34
ostheer: T3: stug and p4
usf: major: jackson and sherman
okw: flak: jp4 and p4
and finally ukf: T3: firefly and cromwell
Main difference FF has 100% chance to hit and penetrate a PzIV (Ostheer and around 100% OKW) at range 60. It also has a chance to completely obliterate it with Tulips.
The unit is simply too effective both VS mediums tanks and Vs Super heavy ones.