Most American fighter aircraft lacked cannon. Consequently firepower was weak compared to most contemporary fighter aircraft.
Most American aircraft used 6 heavy machine guns.
Many Russian fighters used a single 20mm and 2 rifle caliber machine guns or ONE or TWO heavy machine guns, The LA-5 used two 20mm guns.
Japanese Army fighters took most of the war to come up with 20mm cannon, 2 or 4 Mgs was the norm for them and their 12.7 was quite a bit less powerful than the US 12.7mm.
Japanese Navy fighters (Zero) use two low powered 20mm and 2 RCMG for the early part of the war. Later planes got a bit better but it took until 1944 for significant increases in Japanese naval fighters fire power.
Less said about the majority of Italian fighter's guns the better.
Rather leaves the the British and German fighters doesn't it?
Bf 109s unless they were gunboats had one 20mm and two RCMG until the G-6. G-6 replaced the RCMG with the MG 131 which while rather nice weight wise wasn't in the same class as the 12.7mm Browning in power. MK 108 doesn't show up AT ALL until late fall of 1943 and doesn't show up in numbers until when? Fw 190 does have the edge in fire power over the Americans but isn't the most common fighter in German service for most of the war.
It gets down the British having the firepower superiority over the US fighters if one is talking about "most contemporary fighter aircraft".
US fighters may have had weak firepower compared to certain examples of contemporary fighter aircraft but the majority of fighter aircraft used in WW II did NOT have superior firepower to the American fighters.
Especially if you count combat duration or length of time the guns could fire.