That's because mozilla foundation never actually taking donation drive seriously.

Let's consider current situation: currently, mozilla corp allocates significant engineering resource to develop revenue-generating services such as pocket, vpn, and now, AI stuff. What if mozilla never need to try to chase revenue, and instead focus on being an actual foundation, funded by grants and donations? Their expense would be significantly lower.

Let's say mozilla able to refocus development back to firefox and retain 250 highly paid engineers, with yearly expense for salary, benefits and other overhead at ~$100 million per year. That's less than 1/4 of search royalties they got from google in 2020. Now put those $300 million extra money into an endowment instead of wasting it on marketing and other revenue-chasing activities, and start to seriously looking into grants and collecting donations like wikimedia foundation, and in a few years mozilla might be able to amass a huge fund to guarantee independent firefox development for years, or even in perpetuity with huge enough endowment.