I really don't think there's any viable business model for Mozilla. Browser development is too expensive and no one pays for a browser. Google finances itself through ads, Microsoft isn't even developing their own engine anymore and sells it as part of their whole OS anyway.

Other browsers are irrelevant because they are mostly a new UI on top of Chromium.

Donations won't ever be enough unless some billionaire sees internet freedom as important or countries start to see an open web as a part of public infrastructure. Both are highly unlikely, so hopefully the deal with Google doesn't decrease too much with shrinking market share.