Because Mozilla gets money from it and it is something familiar for users.

As a user who knows about "defaults" you're way ahead of everyone else and can probably figure out how to change them. For people who don't care Google is perfect.

Because they figured they'd get more money from sharing ad dollars through Google search than they could do through Bing I would guess. Does anyone know if their Google partnership is worldwide or do they work with Baidu/Yandex in other parts of the world?

It may be the default, but the search engine is fully configurable, with no degredation in functionality.

Right, yet the Google, the evil tyrant, still thinks the default setting is worth paying $160m a year, accounting for over 80% of Mozilla's income.

So is using Mozilla in the interest of Google, or is it not?

I think the users would be disappointed if it was Bing or duckduckgo. Wouldnt they?

They pay Mozilla a lot of money for the privilege?

It actually is most of the funding for Mozilla, at least in 2011:

http://techcrunch.com/2012/11/15/mozilla-releases-annual-rep...