This is a good idea on paper. But it turns out that having a for-profit corporation (as Mozilla Corporation is) accept donations, especially donations earmarked for certain purposes, is understandably tricky from a regulatory and tax standpoint. You can do it, but it comes with lots of rules and restrictions, and constrains the company in weird ways that kind of make sense, since it's kind of similar to money laundering. (And I'm not talking about tax deductible donations, which are a no go for obvious reasons. "Don't pay us $50 for our product, donate $50 and we'll give you the product for free, and you'll lower your taxes!")

The Mozilla Foundation is what you can donate to, and you can do it because it's a non-profit. But it doesn't make Firefox. It owns Mozilla Corporation, which does. And it can't just dump donated money into Mozilla Corp either; regulators are not naive.