As far as I know the source is responsive, when I reduce the width of the iframe to about 700px it does tend to rearrange itself. I still find it odd that it appears perfect in Safari, but not in Firefox
I definitely think it has to do with the way the content is responsive.
If I put max-width 600px on the iframe, things reorganize and there is no scroll, but if I put max-width 800px, they don’t reorganize and I get a scroll