If you see this error, it is probably because a website is trying to display another website without the consent of its owner. This is usually the result of a security misconfiguration.
Websites can use x-frame options or a content security policy to control whether other websites may embed them in their own pages. They are important security tools designed to prevent clickjacking, which is an attack that allows malicious sites to trick users into clicking their links.
To visit a site that has shown this message, you can open the link in a New Tab or New Window in Firefox. Note that in some cases, the embedding page will not work correctly without access to the blocked page. In this case, you will need to contact the owner of the broken site for troubleshooting.