I would swag the Firestone has an older design casing than the Michilen tires. I just took off a set of Discount house brand tires, put on Cooper AT3's and yes, after 8000 miles, I can say I too have gained an MPG. So the case design can make up a difference. Many manufactures like Cooper, Michelin have one case design, but put different tread and rubber compounds on them to work in different situations better.
I do believe what the OP is saying, as this has happened to me other times in the past too.
Marty