Steps and link to replicate the issue would be helpful. What OS? What version of Firefox?

Try to right-click the page with the error message and use one of the choices in the This Frame submenu to open the embedded page.

This happens if you open an external webpage in an iframe and the website prohibits this via CSP rules or via the X-FRAME-OPTIONS header in the HTTP response headers.

The front page of my Wordperss site had been embedded in a scheme to get my Username and Password.

I knew of course that is what it was, and I added some code to the (.)htaccess file to stop embedding.

But now every time I make a small update to a plugin or theme, the page comes up warning me it can't open it due to embedding.

How to get rid of THAT? What is the proper code to add to stop embedding?