Earlier quoted context omitted.
IIRC Firefox (and I assume other browsers) when using privacy/no tracking mode does send fake data..
You're incorrect. I've never seen any browser, on it's own lie about it's user agent. (I can set a custom string and lie with it, but that's not the agent doing it) Do you have a specific / concrete example in mind? Or are you mistaking a feature from something other than a mainstream browser?
IIRC it defaults to a Windows user agent even when you use it on other operating systems.