That has nothing to do with anything. If you're just having a dick measuring contest, Google has contributed a ton to the open source community. Google also provides nearly 100% of funding for Mozilla.
Open source is not about smelly dicks, it's about transparency, reusability, philsophy, politics. It's a lot of things (and they matter, because right now, society and technology are changing fast).
For the average end user, it may not particularly matter that Mozilla's PDF implementation is open source and Google's isn't. However, for people like me who want to implement PDF-related functionality in their web/desktop application, it matters a great deal.
If you're in the US, you may not notice it, but Bing is plain awful for international users. Google has spent a considerable amount of resources improving their engine to serve results based on your location and Google's Search Engine is clearly the best by a long shot. This is also why I'm not using DuckDuckGo.
My guess would be that if the offer is similar, Mozilla would be inclined to stick with Google. I'm sure you know how irritating it is to realise you missed a checkbox and now your homepage and default search are set to Bing - it's probably a user experience Mozilla was happy to avoid.
Plus, there are probably some advantages to having Google 'on side'.
Perhaps they had made an offer. We never would know.
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.
Mozilla has been saying for years that they will use the best engine, and take that engine's money if it's available. If Bing were clearly better than Google, they'd switch and be taking Microsoft's money.
They have said that during the time they are being offered the most money by the best search engine. That's easy. But Google keeps paying more which means Mozilla is after the highest bidder.
That is not how Mozilla's decision making works. There's considerably more to it than such simple economics.
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.