Gmail sidebar loads Calendar no error but sidebar empty/clear/white; Calendar only (other sidebar apps eg Tasks or Contacts load fine); Firefox only (other browsers eg Chrome or Edge no problem.

I suspect this is the Google calendar is written to function with Chrome as they are the same software and uses the same coding that is why they function together. And Firefox isn't part of Google so it would stand to reason there are software differences that works in Chrome but not in Firefox. I always advise those using Google Apps to use Chrome because of the integration of Chrome and Google apps.