This topic keeps coming up. Mozilla is viewed as a bloated entity whose only legitimate activity is maintaining Firefox. No-one cares about the other initiatives they spend money and resources on, and they especially don't want their donations going toward Mozilla's overpaid leadership.
People would like to donate directly to Firefox, but as things stand, this isn't an option. This isn't because Firefox falls under the Mozilla brand, it's because Mozilla choose not to accept Firefox-specific donations.
https://news.ycombinator.com/item?id=24141852
https://news.ycombinator.com/item?id=24129794
https://news.ycombinator.com/item?id=24129926