What happened?
For TestComplete to be able to access the contents of web pages running in Firefox, Firefox security settings must be configured in a special way. If Firefox security settings are preventing TestComplete from accessing a web page when you are exploring it with the Object Spy, TestComplete will show a notification popup message.
What to do?
Disable sandboxing in Firefox as described in:
Web Testing with FirefoxTo disable the notification message
Not to see this notification message in the future, disable the Show Again Flags > Show the "The Web Page Is Not Accessible" message option.