I am having this problem too. Did the typical clearing of cache, cookies, etc...restart...no change. Left navigation loads, top panel loads, just the main calendar data frame never loads. At the very top of the webpage there is a continually moving line across the page from left to right as it is trying to load. Windows 10, Firefox 84.0.2 (64-bit) (does work with chrome)

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.

• Bug 1688211 - Firefox sometimes failed to load Google Calendar

(please do not comment in bug reports
https://bugzilla.mozilla.org/page.cgi?id=etiquette.html)