So I don't really see this being true. Also foundations shouldn't seek to grow but to keep doing why they do.
See how Wikipedia/Wikimedia, while both getting more money and providing almost the same service, still haven't updated how people can actually parse their content but, at least, didn't start selling data.
But I think it's the paid subscriptions that are the main channel of personally contributing revenue towards the part of the organization that develops the browser.
Does EFF do anything in the standards process? They're not an implementer, and they don't employ devs which work on browsers (as far as I know).
EFF does a number of things such as advocate for web standards that don't benefit Google at the expense of other browsers. They get involved in legal action and legislating and they publish privacy badger, an extension that blocks tracking. My understanding is they also worked with Mozilla to stand up Le's Encrypt as a broadly accessible way to access HTTPS.
The users can't pay; there is no way to give Mozilla money for Firefox.
Another issue is that when you get donations, they are usually more than needed, so FOSS devs get to work adding new features which make the product worse.
Consider Whatsapp, after 15 years we can count the added features in 1 or 2 hands. They manage to find great restraint, possibly related to a low engineer-personnel ratio.
FOSS engineer orgs have no chance at staying the course. It reminds me of the sex mantra, if they likes it just keep doing it don't do it harder.
Exact same thing happens with Wikipedia. But the community puts a halt to most initiatives and they have to keep doing spinoffs like wikidata and wikiuniversity and such