A majority of Firefox's funding comes from Google. I doubt they're willing to aggressively market their browser as "the one to use to get around Google's ads". I'm not sure of the exact figures but as I recall, losing Google's funding would be a company-ending event.
In other words, we might need a browser that’s actually an Open Source project, as opposed to something built by company and released with source.
Good luck with that. This kind of approach might work with some projects where you don't need responsive updates and continuous development to keep up with changing standards and new vulnerabilities being continuously found, such as vim, but for a really big software project I'm not aware of this working at all. The Linux kernel is maybe comparable in size and scope, but it's developed by a bunch of corporate developers too, though they're scattered across different companies that all need the kernel. A web browser isn't really like a kernel; how many companies are going to contribute developers to a project like that?