Secondly, the reason so few users donate to open source projects is because these projects are so poorly marketed to potential supporters. That’s why a sophisticated organisation like Mozilla is so well placed to sell the stories behind some of these projects.

This is definitely a good point.

the percentage of users that click on ads and shopping is also very low.

You'd be surprised. I've worked in ad tech. Retargeting ads (where you see ads for items you've viewed in the past) and abandoned cart ads (which you see if you add items to your cart but never check out, sometimes with a discount coupon attached) have very good clickthrough rates. Targeting based on customer list performs pretty well too.

In any case, I really doubt they could make even 1% of what they currently make with the Google deal. AFAIK they make around $400 million per year from that deal: https://www.pcmag.com/news/mozilla-signs-lucrative-3-year-google-search-deal-for-firefox