You can donate to the foundation. I'm not sure why that would turn out any different -- even if you paid a subscription for Firefox directly, what would prevent Mozilla from using that money for their other products?
The problem with optional subscriptions is that most people will never opt-in. If subscriptions are made mandatory, the general public will be driven to one of the other major browsers.
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.
However, money can go the other way. Paying a subscription might end up just sending at least some of the money to the Foundation, rather than Firefox (or even Rust) development.
The problem with optional subscriptions is that most people will never opt-in. If subscriptions are made mandatory, the general public will be driven to one of the other major browsers.