Earlier quoted context omitted.
When it's installed automatically it doesn't have the same intent behind it that justifies changing the setting.
Windows isn't installed automatically. Let me lay out the analogy more explicitly. I install Firefox, and specifically choose for it not to be my default. I later download an updated version and choose default settings in the installation. Firefox then sets itself as the default browser; it overwrites my previous choice in favor of the default. I install Windows. I specifically don't set explorer as the default. I in…
Firefox will only change things if you specifically get a Firefox installer, where it thinking that's your intent is understandable.