I've recently seen quite a few reports of issues with accessing Google Calendar, so it looks that something is seriously wrong. You can check the Web Console for possible error messages. *https://developer.mozilla.org/en-US/docs/Tools/Web_Console You can try to hide and show this content to see if that has effect. There has been a bug report created where you can follow the progress. *[https://bugzilla.mozilla.org/show_bug.cgi?id=1688211 Bug 1688211] - Firefox sometimes failed to load Google Calendar (<i>please do not comment in bug reports<br>https://bugzilla.mozilla.org/page.cgi?id=etiquette.html</i>)