Despite its emphasis on protecting privacy, Mozilla is moving towards integrating ads, backed by new infrastructure from their acquisition of Anonym. They claim this will maintain a balance between user control and online ad economics, using privacy-preserving tech. However, this shift appears to contradict Mozilla’s earlier stance of protecting users from invasive advertising practices, and it signals a change in their priorities.
But you just said
It is an option.
Clearly it isn’t working well enough for them.
They would have had to build that infrastructure. I’m not saying fundraising is easy. But it’s possible as proven by wikipedia. They could have cut Google loose 10 years ago and said "we’re going to use our runway to try to put together a wikimedia foundation style fundraising operation. I don’t think they can do it now because the trust, goodwill and quite frankly, userbase is gone.