iFrame with pdf doesn't work in Mozilla

Hello,
I am trying to set an iFrame card that open a .pdf file in Mozilla.

If I put the path directly on the iFrame card the browser open it in a new tab and download the .pdf file.

If I put an iFrame inside an HTML page and open it in the Mozilla browser eveything works.

Instead if I put the HTML page in the iFrame card, the card show nothing and the browser open the pdf file in a new tab.

I have made a lot of search but nothing that can help to solve it.

Do you have any advice?

Thank You
Regards
Matteo