Comment by npteljes
I think that's because those everything else are products with an opaque structure, and Mozilla, and for example Wikipedia, are more transparent. Really highlights why some people don't open up, either themselves, their source code, or their organizational structure: it's just inviting endless criticism.
Adding to the point, donating to Mozilla (or Wikipedia) is optional, and paying for a product is not, legally. So if I'm buying clothing, it's whatever, I need my clothing, and the price is just the functional gateway of getting it. But in case of a Mozilla donation, I'm trying to do something good in the world. And if I discover that it's wasted, then I'm not just getting nothing - I am worse off, because I supported a bad cause.
This has been part of my conclusion too.
There's an irony that in providing people the option of not paying, you are also inviting them to find flaws in your organization to avoid paying. We are all aware that Microsoft sucks, yet there's never any doubt that you'll have to pay for a 365 subscription if you're a serious business. At the same time we'll also gladly accept that small companies don't donate to the Linux foundation, because they have to pay their bills.
By using the control we advocate for (forking projects, reducing funding, etc) we only harm the projects that afford us that control. Not paying Mozilla does nothing to reduce the control of Google over chrome. It only hurts the one browser that gives you the choice.