Superior training and tactics and not necessarily the machine. Also, in those early winter war days exactly what were the Finn's up against? I do know that Stalin purged his military so badly that they had a large deficiency in leadership, motivation and training. Many Finnish pilots racked up enormous scores on the Finnish/Soviet front by using formulaic defensive tactics against Soviet aircraft. The default tactic was the four-plane "parvi" with a pair flying low (but visible, not too close to the terrain) as the bait, and a pair flying high to dive on the eventual interceptors. In the long run, the Soviet Air Force on the Finnish front never developed an efficient approach to counter this tactic. According to some reports, this tactic also inspired the German Luftwaffe's kette.
Upon further inspection the export B-239 is little more than a F2A-1 with a two extra guns and a 950hp rated R-1820-G5 instead of a 940 or 950 hp rated R-1820-34. Once in country the Finns added armored backrests for their pilots, metric flight instruments, the Finnish Väisälä T.h.m.40 gunsight, and four .50 in (12.7 mm) machine guns. The top speed of the Finnish Buffalos, as modified, was 297 mph (478 km/h) at 15,675 ft, and their loaded weight was 5,820 lb. Not a tremendous difference between the US navalized version and this version.
So how was it we got our MiG kill ratios back up to snuff during the Vietnam conflict? We took on inferior machines such as the MiG-19 Farmer with the state of the art Phantom II and still from 2 March 1965 to 1 November 1968 we saw almost 1,000 U.S. aircraft losses in about one million sorties. The advent of Top Gun did not change the equipment, except to give the Phantom II an internal cannon. It provided for superior tactics and training.
This also brings to mind the subject of Jimmy Thach and his weave that confounded the superior Zero. The B-239 was not a superior weapon of war ....the Finnish pilots were.