shop.fairphone.com works fine. The hamburger menu on the forum sometimes disappears, a refresh fixes that most of the time. I think they should update their theme soon, it’s not getting better with every Discourse update. Using Firefox as well, but on a Pixel 3 with Android 12.
It should work on mobile too, go to this menu and then check the Theme option. Set that to “Fairphone original”, which indeed has the least issues. But would’ve been great if their custom theme works again. Because it does look nice.