What incentives does the for-profit (that's owned by the non-profit) have that a non-profit without a for-profit subsidiary wouldn't have? Both aren't able to maximise revenue for shareholders, and both will always have the option to pay their leaders extravagantly.

And as a well-paid programmer, I haven't been known to donate $100 a year to software projects. As a conservative estimate, let's say Mozilla could run Firefox at one-fifth the current budget, that would still mean we'd need a million people like you that would continue to do so even if, say, the most-often-voted-for feature request is misinterpreted, or changing a "view all tabs" icon suddenly pisses off a significant portion of them enough to stop their donations.

And even if that happened, it's not clear that that would necessarily lead to gaining market share on default browsers or ones that get heavily promoted through search engine homepages or shadily bundled with installers. Which would still mean more and more websites would start to ignore it, which would mean web compatibility would continue to get worse and worse.