Straightforwardly, nobody with the resources to build and maintain a browser could ever be trusted to do so, as they get those resources from somewhere, and it's going to be somewhere unacceptable (ads, trackers, Google, taxes, selling the browser or addons/features as a product, hostile nation states).
These things are far too complex and expensive to be produced as they should and as most FOSS is: for free, by a group of individuals who could fit around a breakfast table and don't answer to anyone but each other.
Users tend to talk about mozilla like it's holding firefox hostage or something, but really the main gripe they have is building and maintaining browsers is beyond the scope of an individual. THEY couldn't fork firefox, but it's not impossible for someone with resources and connections to MAKE an entity that does. It just... hasn't happened again. Mozilla is a pretty insane freak-of-nature in the business world, and there's honest frustration about that which grows into "Firefox is doomed".