So my statement was a bit too short and not quite accurate. For a reference, the closest thing I could find within 10 minutes was this mailing list thread:
https://groups.google.com/d/msg/mozilla.governance/flhYG39Oc...It starts off specifically about the removal of Yandex as the default for the Russian localization of Firefox, but touches on the greater question of why Google vs anyone else given our differences on tracking users and such. For context, the Harvey mentioned is Harvey Anderson, who was Senior VP of Business and Legal Affairs at the time.
My personal summary would be something like this: Mozilla has a mission defined in the manifesto. We have a ton of projects that are supported by the funding provided by Google and other search engines. There are several factors being balanced in the choice of search engine: usefulness to users, respectfulness of user privacy, and sustainability of the project, to name a few.
JohnTHaller's got it right: Sure, we could switch to DDG and max out the respectfulness of user privacy aspect. In return, the usefulness to users goes down[1], and the sustainability plummets. We'd be sticking perfectly to our ideals, but shooting ourselves in the foot (assuming we want to continue doing all the different projects we do).
In choosing Google, the leadership of the project decided that Google as search provided offered the best balance of usefulness, respectfulness, and sustainability. You can argue that they're really bad at respecting user privacy[2], but they'd argue all the alternatives offer an even worse balance. They think that we can provide a greater net good to the world by going with Google than we can by going with DDG or someone else.
Mozilla tends to favor pragmatism, IMO, and sometimes that means focusing on sustainability (a nice word for making money to pay for our other activities). Hence why we considered directory tiles, among other ideas, so that we can pay the bills and keep making the web better.
(I'm intentionally ignoring side-arguments like "Why does Mozilla need so _much_ money?" or "Why do VPs get to make decisions without consulting the community" because one comment isn't enough to address a complex topic like this :D)
[1] IMO, I find DDG's results to be worse than Google's, and although I'm fine with that in return for their other great features, not all users are.
[2] And I'd mildly agree, not being super-knowledgeable about the specifics personally.