Obviously, a corporation (like Google, MS, Apple, etc) can't be trusted because they have too many conflicting interests. The MS monopoly lawsuit was about browsers; how people connect to the internet is a big huge deal and there's a lot of money and power to be gained if you become top-dog in that space.
Just ask Google, whose dominance of the browser space with Chrome has massively shaped the way to web works today, making it much friendlier to invasive DRM, micropayment schemes, invasive advertising, and many other consumer exploiting features.
So you need an independent source for a browser, but browsers are big and very complicated and need strong security and on and on. It's neither easy nor cheap, but why would the average person want to pay for an independent browser when MS, Apple, and Google will all give you a great browser "for free?"
There is no real answer to this under our current economic model. Donations and fund-raising are patches on a broken system, usually requiring additional support from advertisers, corporate donations, influencing foundations, and so on. If the money isn't coming from consumers buying or donating, then it's coming from someone with money and a vested interest that their money is buying influence for.
The Internet was built on public money from university grants and military funding. But the internet was stolen from the public decades ago and sold to big capital. Until we fix that, Mozilla is just another in a long line of past and future downsizings as the corporate world continues to pillage and privatize the Internet.