Iframe blocked by Firefox even if the origin is on the same domain (GLPI)

I've got a landing page on a domain : www.extranet.domain.com On this page I want to

<iframe allow-same-origin="true" src="https://anotherdomain.domain.com/page" frameborder="0" style="width:100%;height:100%"/></iframe>

Instead of getting the Iframe, I got a security issue from Firefox saying that I need to open the website in a new window (cf image https://i.stack.imgur.com/0veFo.png)

Thanks for reading ! <smile!>

UPDATE :

The login page of the website (GLPI) on the "anotherdomain.com" works. But When I log-in I got the error !