Here's a sort of thought experiment, which I really don't know the answer to. How much of Firefox development (by hours, lines of code, commits, pick your metric) is done by Mozilla employees as part of their day jobs? And how much of that development is ~useless for the maintenance and improvement of the core browser? (Stuff like Pocket and the "discovery" feature on the new tab page, all the time spent on the URL bar changes, etc etc.)
It might turn out to be the case that most, or even all of core Firefox development could be done by the community. At the very least I'm willing to put out there that maintaining a web browser does not require half a billion dollars a year. (It's rather hard to imagine that 100 full time devs making $250k a year each couldn't handle basically all of the core development, for example.)
The problem with Mozilla is that it's a business, for all intents and purposes. With an open source project, as long as ongoing development is financially maintainable you don't have to care about marketshare. Mozilla cares about marketshare because it affects how much Google will pay them for search traffic (and how much they get from in-chrome advertizing).
When Oracle proved derelict in its duties managing the development of OpenOffice, the Document Foundation was formed, largely by people who were already spending their time on OpenOffice. They forked OpenOffice into LibreOffice almost overnight. Major longstanding pull requests were quickly merged. New people who previously hadn't wanted to contribute stepped up to support a community project. And the result has been all to the good! The Document Foundation still manages the project, and in 2019 did so with a total income of less than a million Euros.
To me this is proof that large open source software projects can quietly evolve into two camps: one is the employees of a governing corporation, and the other is the community. And the community might be doing most / all of the core work required to keep the project running, and the employees are mostly doing stuff that benefits the company. And suddenly there's a community fork - and it just works, without any hitches. I don't know if this would be successful in the case of Firefox, but I expect (and hope) that people would try if there was ever a hint of Mozilla abandoning it.
(For another example, consider KDE e.V., which manages an enormous open source project on a budget of under 600k Euros a year as of 2018.)
