The difference is that the Foundation
legally cannot spend their tax-free money paying the salaries of employees of a for-profit entity, to develop a product that pulls in significant revenue.
It's a matter of "can't", not "won't". Subscription revenue would go to Mozilla Corp directly and therefore has no restrictions, because it's taxed as income. It at least has the possibility of being spent on Firefox development even if it actually goes to Rust maintainence or whatever else.
