It's an interesting test but I noted a couple of things. The Firestone is an R-1 and the Michelin is an R-1W. So for a 18.4-42 tire there is a difference in lug height 19/32nd or about 5/8" on a brand new tire, 50/32nds for the Firestone and 69/32nds for the Michelin. That difference in lug height could make a difference in pulling force when new due to lug flex, depending on conditions. But you should get longer life with 5/8" more lug on the Michelins.

I don't know what size tire was on the tractors, I just picked a size randomly. I have nothing to gain with either tire, I just think these tests are interesting and it can be fun to see if there's a reason why one might win over the other. You can bet if the ground was softer or wetter the R-1W might be the better choice.