Ford told Firestone what type of tire they wanted.
They wanted a tire that would make a large 4 wheel drive vehicle ride as smooth as a car. To do that you have to use a softer compound rubber instead of a harder tire that you would use for a truck this heavy.
The result is you get a smother riding tire, but you loose the stability you need in a vehicle this heavy.
The blame falls on both Ford and Firestone, they should both be held responsible