Firstly, that's not a scaling problem, you're talking about poor uptake.
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.
Thirdly, the percentage of users that click on ads and shopping is also very low. Particularly amongst more technical users.
Fourthly, this plan would actually drive users to Firefox. If Firefox is promoting donations for say, LibreOffice, then they would naturally have an interest in promoting Firefox.
With the advent of enshittification, free-as-in-beer tech is dead. I think people are realising that things need to be paid for. It's very defeatist to just say "no one contributes to open source". Why not try to find the format within which people might contribute?