This sadly seems to be a Unity bug. On Firefox on Windows, this shows up in the browser console:
Uncaught DOMException: The operation is insecure. fcdd8955-fe3b-4477-9e70-840ae811a7dc:8
UnityModule blob:https://v6p9d9t4.ssl.hwcdn.net/fcdd8955-fe3b-4477-9e70-840ae811a7dc:8
processWasmFrameworkJob https://v6p9d9t4.ssl.hwcdn.net/html/4278477/Build/UnityLoader.js:4
onload https://v6p9d9t4.ssl.hwcdn.net/html/4278477/Build/UnityLoader.js:4