A couple of a days ago, I managed to finally get around to installing FreeBSD in a virtual machine and tried reproducing this issue using the same user agent, but to no avail. Even after using the same user agent, it was loading the desktop website.
I think this might be an issue with an older version of Mozilla Firefox. I’m running the latest version (96.0.3), which might’ve fixed the issue altogether, so try updating to that in the meantime. I’ll try installing an older version (95.0) to determine this issue once and for all. Just give me a couple of hours.