You can change the header, but browser developers are not that dumb and they added properties like "navigator.platform" which do not change and immediately give you away. Consider also writing a browser extension to patch these properties. Also, I think that DRM module (widewine), that is bundled with browsers, also can report the actual software version. Sadly it is undocumented so I don't know what information it can provide, but I notice warnings from Firefox about attempts to use DRM on various sites like Yandex Market.