Allow content documents (web pages) to detect my Firefox addon

Question

In my Firefox addon I\'m looking for a secure way to let content code detect the presence of the addon itself. Ideally what I\'d like to end up with is allowing content code to

Answer 1

Adapted from here (but using a getter to make the my_addon value read-only)

// contentWindow is the window object of a contentDocument being displayed
var s = new Components.utils.Sandbox(contentWindow);
s.window = contentWindow;
Components.utils.evalInSandbox("
  window.wrappedJSObject.navigator.__defineGetter__('my_addon', function(){ 
    return true; // or whatever we want its value to be
                 // (note: this is unprivileged code!)
  });", 
  s
);