Those are two separate programs and maybe if you have the page opened in Firefox the website editor active it update the files because it's opened currently? I just sounds like it can run if you browser and editor is on the same site. Did you try to launch the site in Firefox just by itself and the website on another browser to see what happens? Just my hunch there is a open file conflict from the site itself.

What OS? What version of Firefox? Show the issue. Do we have to have an account to replicate the issue? Do you have the same issue with Firefox in Troubleshoot Mode?

This happens if a web page opens an external page in an iframe and the website prohibits this via CSP rules or via a X-FRAME-OPTIONS header in the HTTP response headers. You can right-click the frame area with the error message and use "This Frame: Show Only This Frame" or "This Frame: Open Frame in New Tab" to display this page.