There's some gripes around that, actually, as donations don't flow to Firefox but to the other stuff the foundation does with the profits they earn from Firefox. In fact Firefox development is done by the corporation, not the foundation, so they can't even send money there (at least I assume you can't funnel money from non-profit to for-profit corporations in the US, that would be nonsensical even for their legal system). It might fund Firefox indirectly, though, e.g. awarding funds for a FLOSS SSL library which Firefox then uses but stuff like funding a UI framework or such probably won't fly because it's not web.

It's not that the stuff they fund is bad, it just ain't Firefox. Also they have quite some administrative overhead (30%). About 1% of what they spend on that stuff comes from donations, the rest is income from, primarily, Firefox, primarily by selling the default search engine spot -- different ones depending on market.