Soymilk for drinking vs. soymilk for tofu making
Your title, your problem and your question (btw question is general but complicated) while on the same subject matter are not the same.

In regards to your quest for firm and extra-firm tofu, the OP doesn't state much about the process after the coagulation.  Which similarly to cheese making is the (arguably) the most crucial factor in the final texture.  firm and extra-firm tofu is pressed with heavier weights, and for longer periods of time.

The choice of coagulent (nikari, gypsum, acid, etc), temperature of the milk and the mixing method can also affect the texture of the final product.