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
Realistically, the corporation makes so much more money from Firefox search deals versus foundation donations that this shouldn't be necessary. But as long as the Mozilla leadership wants to screw Firefox they will find a way. For example, by removing an equal number of engineers from Firefox development on the corporation side as the foundation hires.