The OS does set itself to do OS things.
OS shouldn't set your browser.
OS shouldn't set your browser.
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 install an updated version of Windows and choose default settings. Windows overrides my previous choice.
Firefox will only change things if you specifically get a Firefox installer, where it thinking that's your intent is understandable.