As mentioned here https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Content-Security-Policy/frame-ancestors, CSP: frame-ancestors is supported from Firefox 33. However, it seems doesn't work. I am trying to embed a 3-party site into our page using an iframe. The 3-party site did whitelist us using these headers - Content-Security-Policy: frame-ancestors 'self' https://*.ourdomain.com - X-Frame-Options: SAMEORIGIN It works fine on Chrome, but not Firefox. I am using Firefox 79. Is there anything wrong with our headers? Thank you!