Obviously, since it sounds like it's not a consistent issue, it might be hard to troubleshoot it. However, my first guess would be that it's something to do with a content blocking add-on that you have installed on Firefox, like uBlock. It's possible that one of them is blocking content that prevents the rest of the page from loading properly.

I use Google Calendar nearly daily and I've never had the issue you've described.

Yes, it's not reproducible. I think it started a few months ago. I'll try safe mode the next time it shows up.

Good catch! The problem was uBlockOrigin extension. I reported it to their Reddit. Thanks!

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