I've never been a fan of Firestone tires whatsoever. I've had nothing but bad experiences with them on cars I've purchased that had them. My 2014 Ram was no different. The stock Transforce tires were down to the wear bars @ 28K miles less than 5% of that towing miles.
But, I have also seen some good reviews and have the same question as the OP.
I think there may be two types of the same tires. One that is the "regular" and one that comes on new vehicles. I think this because as I was shopping on Tirerack they have Transforce listed and then Transforce tagged as "Original Equipment" for much less. If that is the case I'd be surprised a manufacturer would pollute their brand but maybe they do it that way?