Mozilla is definitely NOT equal to your average open-source project like Debian or LibreOffice. The development on those projects is done entirely by volunteers. Maybe some of those "volunteers" are paid by companies to be contributors, which is the case with the Linux kernel.
But Mozilla is a large company. It may be a nonprofit, but there are HUNDREDS of millions in Google money funding the company and paying salaries for the 1,000 (??) or so employees, including, as mentioned above, a very well-compensated C-suite.
Firefox and whatever else Mozilla produces is funded with this very, very large pot of money. Could they run the Mozilla operation on $10 million (or $20 million) a year and invest the rest ($90 million to $390 million) a year in an endowment that could ensure Mozilla's independence in perpetuity? They could do that, but like many nonprofits, the "profit" goes to a large number of well-paid people. I can't think of another open-source project with such enormous funding that isn't part of a for-profit company.
Red Hat isn't asking individuals for money to fund Fedora or RHEL development.
I don't know what crazy universe a company -- even one hiding behind the framework of a "foundation" -- gets $100 million to $400 million a YEAR from Google -- and still has the chutzpah to ask individual users to make a contribution.
I love Firefox, and I absolutely agree that browser diversity is important. I use Firefox daily. But Mozilla is a huge outlier -- a "nonprofit" dragging in hundreds of millions and spending it. Do you think Debian collected more than $1 million last year? I'm not sure it did. It sure didn't get $100M or $400M. Mozilla is a huge company. It just doesn't have shareholders.