How can I fool a site that looks at the JavaScript object 'navigator' to see that I'm not on Windows?

var fakePlatformGetter = function () {
  return "your fake platform";
};
if (Object.defineProperty) {
  Object.defineProperty(navigator, "platform", {
    get: fakePlatformGetter
  });
  Object.defineProperty(Navigator.prototype, "platform", {
    get: fakePlatformGetter
  });
} else if (Object.prototype.__defineGetter__) {
  navigator.__defineGetter__("platform", fakePlatformGetter);
  Navigator.prototype.__defineGetter__("platform", fakePlatformGetter);
}

Since you can't directly set navigator.platform, you will have to be sneaky - create an object that behaves like navigator, replace its platform, then set navigator to it.

var fake_navigator = {};

for (var i in navigator) {
  fake_navigator[i] =  navigator[i];
}

fake_navigator.platform = 'MyOS';

navigator = fake_navigator;

If you execute this code before the document loads (using GreaseMonkey, an addon or a Chrome extension), then the page will see navigator.platform as "MyOS".

Note: tested only in Chrome.

about:config - > general.platform.override