If you see this error, it is usually caused by a misconfigured website that is trying to display another website without the consent of its owner.
Websites can use x-frame options or a content security policy to control if other websites may embed them on their own pages. This is an important security feature to prevent clickjacking, which is an attack that allows malicious sites to trick users into clicking links on a site.
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.