This is true.
> But still, MoFo contributors, paid or not, do not work on Firefox.
This is not true, based on what I've read about it. Do you have personal experience with these orgs that suggests otherwise?
Regardless, nothing is stopping Foundation funds from being directed to Firefox development. If someone gave them, for example, $1M that could only be spent on Firefox, they could pay Corporation or an external consultancy to contribute to the open-source Firefox repositories.
This is already happening, either through Foundation or Corporation. One of the biggest Servo contributors works for a FOSS consultancy.
There are corollaries to what I'm describing in most large nonprofits in the US. You get money that a donor requires you to spend in a certain way, and you spend that money that way. If you can't do it with in-house people, you give it to consultants.