Thanks for sharing all those details – definitely helpful.
I opened your site and definitely was able to see the issue. Did a bit of research and it looks like this is a known bug (reference). Super specific FF bug that only affects % based margin / padding of child elements in a flex container.
As a quick workaround you can use vh instead of % though (GIF)
Hope that workaround helps. I’ll also check out that other forum post of yours — can you share a link? 