If Microsoft were willing to provide pay as much or more, why aren't they doing it already to make Bing the default search engine?
Plus, there are probably some advantages to having Google 'on side'.
I think this is sufficient to show that they're at least thinking about it.
We don't even know the details of the Mozilla-Google deal. For all the openness that Google and Mozilla appear to proffer, the agreement between them is a secret.
As a non-profit, we are here to create a public good. It's actually goods, plural, and services too. Firefox and other Mozilla products and services exist to promote the Mozilla mission, not to enrich the Mozilla organization.
Yes. Sustainability is certainly a factor in some of our decision making. Mozilla would be foolish to ignore significant opportunities to increase its mission impact. But revenue for sustainability is not the sole or even the dominant factor in any major product decision at Mozilla.