The T34 was good enough to survive until the Korean War where it was faced by much improved UN tanks, beyond the Firefly (although Shermans did see use in Korea). In fact the Firefly was largely a British adaptation rather than American. The beauty of the T34 (76 or 85) was its simplicity. It was designed for mass production and for use by largely unsophisticated peasants. Their training, compared to the already experienced panzer crews, combined with leadership which had survived the purges and lacked initiative and talent for mobile warfare, made high losses inevitable while they learned their trade. This contrasted with the US which had ultra efficient and sophisticated production techniques and generally more educated crews and some degree of continuity in its tank leadership.
I do not claim that the T34 was the "best" tank of WWII, it was certainly the best at the time of its introduction, but it was definitely the tank which made the most notable impact on the war. If I was sent off to war in a WWII battle and was given the choice between a T34/85 and a Sherman (Firefly or otherwise) I know which I would be happier to entrust my ass to.