I've been reading about tank warfare during ww2 recently, It seems like the Firefly was a poorly done conversion, It was essentially just a artillery piece put on it's side that was stuffed inside a turret that couldn't accommodate it properly, It had major problems with being able to observe targets after firing, It also had accuracy issues, Especially with APDS ammo. and they had to remove the hull machine gun. They were issued as the "troop" leader's vehicle so it was easier for the enemy to tell who was in charge. And the armor penetration advantage doesn't seem to matter much tbh, Because the number of tanks the M3 75mm gun couldn't penetrate made up less than 10% of the total german tank force. And since war doesn't exist in a vacuum there were already a huge number of weapons available that could punch right through a heavy German tank without these downsides, The only real advantage i can find is that it shared ammo with the towed guns and could be produced in Britain, But it seems more like a "quick fix" that was quickly discarded, As both the A30 "Challenger" and the Sherman C "Firefly" were removed from service before the war even ended.
It should be noted that the German tank fleet, for the most part, was nowhere near as impressive as their reputation has made them. By the tale end of the war (when the US was getting involved) not only was Germany running out of tanks and supplies to keep them running, but most weapons the Allies were fielding were able to stop them.
To add on to that, most of the myth's around the Sherman comes from Belton Cooper's Death Traps, which has been thoroughly discredited in academia. One myth that survives is the Sherman was an inferior tank and was constantly outmatched by other tanks in combat. The problem with this is that the Sherman isn't meant to fight them (though mind you, the armor of a Tiger I would fail against a Sherman's cannon at distances of 1000 yards or less, and don't get started on the Panzers), as the Sherman was an infantry support tank.
Its primary goal was to assist US infantry in its movements, not fight prolonged tank battles against Panzers. In armored formations, the Sherman's task would be to bait German tanks into American AT weapons, something it did phenomenally. Tank destroys of the US XII Corps reported 125 German tanks killed to only 25 losses! A 5:1 ratio.
And even past that, the Sherman was far better built, cheaper and lighter than the German counterparts. They were exceptionally reliable, which is why they were used up into Vietnam, and were exceptionally cheap for the time as well. The problem with a lot of one on one tank comparisons is that US tanks would simply not be in one on one with a German tank in the first place, doctrine and design meant they were never meant to be. America used its industrial capacity to simply overwhelm the Germans, even when they reached technological parity and then surpassed them.
And the speed, of course, can't be understated. Sherman's were exceptionally fast, and could circle German tanks with comparative ease, which means even though the Sherman could hypothetically puncture most German front armor, they really didn't have to.
Sources:
Zaloga, Steve. Armored Champion: The Top Tanks of World War II. Mechanicsburg: Stackpole, 2015. Print.
Zaloga, Steve. Armored Thunderbolt: The US Army Sherman in World War II. Mechanicsburg, PA: Stackpole, 2008. Print.