I don't see why so many people are against Mozilla using donations how they see fit. They need to diversify because relying on a single project, a free browser, in a single market segment which is saturated with free browsers, is a massive risk. Firefox's survival mostly hinges on Google's money, which of course Google have a massive interest in (they need a competitor to Chromium). However if that were to change, Firefox is going to struggle even more to keep up.
So, in my opinion, Mozilla need to diversify with alternative projects that could bring in money and reputation, and maybe even through integrations users for their other projects. To be able to do that, they need flexibility on how to spend the donations. To see what could happen if there are strict limits on what funds can be used for, check out Atlanta's MARTA system. They have to split incoming funds evenly between operational and capital expenditures budgets, which results in bullshit like them having the money for shiny new trains but not enough for basic repairs. Mozilla probably don't want to find themselves with tons of money for Firefox, but not enough to keep the lights on in general (legal, office, C-suite, other projects).
So I see where you're coming from, but it's just unrealistic.