The use for donations could be for a single person whose job is to check the upstream code for any antifeatures (telemetry, ads, product placements, online service defaults, Google as paid default search engine, etc.) not in the user's interest and revert them, as well as bundling any useful extension like uBlock Origin and verifying them.
That needs minimal effort compared to building a browser, because it doesn't involve doing any of the hard work, but just removing code that serves to line the pockets of those doing most of the work at the expense of the user.
Do I understand correctly that you believe Mozilla doesn't currently have the resources necessary to do that from their $500MM in annual revenue? It sounds like you are talking about an ombudsman or something, which highlights my point here, which is that these are philosophical criticisms disguised as commentary on raising revenue.
Also the mission you are describing sounds like something that you might expect from a Chromium browser that has to regularly revert Google-driven changes. At Mozilla, they already own the browser and they could account for this in their ground-level philosophy.
They could, but they don't want to do that because they get paid by Google to not do it or because those actions get them money in some other way (from advertisers or whatever), or because they think only power users like some features.
Firefox publishes their 990 form which discloses all their sources of revenue and Google does not pay Firefox for any of the things you described. Also, it feels kind of nonsensical to suggest that it would have a development strategy of building out their ad tech and simultaneously reverting it, and I don't see how explanations about them wanting or not wanting to do it make that proposed approach for any more sense.
They pay them for making Google the default search engine, and it is hypothesized that the payment may also influence them to not provide ad-blocking by default and possibly other things that are not beneficial for Google's business.
By whom and on what basis? Those are non-optional questions that should have strong answers as preconditions to you posting about it, if the objective is to offer something more than simple bullshitting (in the Harvey Frankfurt sense of indifference to truth).
This also doesn't answer like 90% of my concerns from my previous comments. Who has ever intentionally had a software development approach of having one team develop features and another person revert those features, working in tandem? And why would they need donations that are 0.20% of what they already get in revenue to finance it? I feel like you're just improv riffing here.
That needs minimal effort compared to building a browser, because it doesn't involve doing any of the hard work, but just removing code that serves to line the pockets of those doing most of the work at the expense of the user.