It is fair to say that the approach I have suggested wouldn't generate as much revenue as force-bundling something to every user of Firefox. But it would generate revenue. Diversification is really possible because if you offer some monetisation component that a user voluntarily opts into, you can experiment with a lot of such things that would otherwise be controversial. (For example, Firefox could tie-up with some ad company that uses some Mozilla tech like "privacy-preserving ad-measurement" that offers to share revenue with the user if they allow ads on the "partner" sites they visit - some % of users would be willing to try it out to make money and / or to support Firefox).

Of course, before they do all that, they should really divert some of the 100's of millions of dollars they have into making a Firefox actually better - faster and less resource consuming. Developers have long complained that despite the money, 20+ years of development, the codebase isn't modular and Gecko still isn't available as a stand-alone module that could be used in other application development or to even make other browsers.

Those working for Mozilla / Firefox need to understand that they have lost substantial trust with their users for some genuine reasons (and not just bad PR from their competitors), and that is one of the major reason why their user-base is dwindling. The way to fix that is to first repair that trust. Reinforce the values that Firefox marketed to its users.