I think the assumption is that you have to be at least technically literate enough to know that you want Firefox and purposefully download it. And if you're that kind of person, you probably also know the difference between Yahoo and Google and probably actually care.
I guess a lot of people care about their browser being Firefox more than they care about the search engine? I once used Yahoo for a few months (also Bing and DDT). It's pretty good IMO. Not everyone would switch to Google immediately.
Every time someone started tracking people and do bulk data collection in the past century, it ended up with people being gassed, put into siberian camps, etc.
That's why I put "default" in quotes. I meant the browser you choose to download when you get a new laptop. Not everyone thinks Chrome is the best browser out there.
