See attached Firefox and Brave images of Google Calendar for example of what is missing in Firefox (note these are two different Google accounts).

Do a remove and then close the browser and reopen the browser and reinstall the Addon again? And did you make sure your not using a older version of the addon.

Hmm, Google does conduct UI experiments on its users, but if this is one of those, it's not very useful!

If a site is generally known to work in Firefox, these are standard suggestions to try when it stops working normally:

Double-check content blockers: Firefox's Content Blocking feature and extensions that counter ads and tracking may break websites that embed third party content (meaning, from a secondary server).

(A) Do you see a shield icon toward the left end of the address bar, near the lock icon? More info on managing the Tracking Protection feature in this article: Content Blocking.

(B) Extensions such as Adblock Plus, Blur, Disconnect, Ghostery, NoScript, Privacy Badger, uBlock Origin or uMatrix should provide toolbar buttons to manage blocked content in a page. There may or may not be a number on the icon indicating the number of blocked items; you may need to click the button to see what's going on and test whether you need to make an exception for this site.

Cache and Cookies: When you have a problem with one particular site, a good "first thing to try" is clearing your Firefox cache and deleting your saved cookies for the site.

(1) Clear Firefox's Cache

See: How to clear the Firefox cache

If you have a large hard drive, this might take a few minutes.

(2) Remove the site's cookies (save any pending work first). While viewing a page on the site, try either:

• right-click (on Mac Ctrl+click) a blank area of the page and choose View Page Info > Security > "Clear Cookies and Site Data"
• (menu bar) Tools > Page Info > Security > "Clear Cookies and Site Data"
• click the padlock or "i" icon in the address bar, then the ">" button, then More Information, and finally the "Clear Cookies and Site Data" button

In the dialog that opens, you will see one or more matches to the current address so you can remove the site's cookies individually without affecting other sites.

Then try reloading the page. Does that help?

Testing in Firefox's Safe Mode: In its Safe Mode, Firefox temporarily deactivates extensions, hardware acceleration, and some other advanced features to help you assess whether these are causing the problem.

If Firefox is not running: Hold down the Shift key when starting Firefox. (On Mac, hold down the option/alt key instead of the Shift key.)

If Firefox is running: You can restart Firefox in Safe Mode using either:

• "3-bar" menu button > "?" Help > Restart with Add-ons Disabled
• (menu bar) Help menu > Restart with Add-ons Disabled

and OK the restart.

Both scenarios: A small dialog should appear. Click "Start in Safe Mode" (not Refresh).

Any improvement?

WestEnd said

Did remove and then close the browser and reopen the browser and reinstall the Addon again? And did you make sure your not using a older version of the addon.

I think the issue is with the built-in website features at

https://calendar.google.com/calendar/r

They are loaded by a script a split second after the main controls, so there may be a content blocking, cache, or cookie issue.

I notice a notch on the Control Center 'i' icon. This indicates that you made changes to permissions from the default. You can click this icon to see what changes this is about.

You can possibly clear the Site Preferences to remove this data, but this resets the permissions for all websites.

Hello everyone and thanks for all replies and help. As I've been really busy since posting I've not had time to act on any of the generous and thoughtful suggestions - THANK YOU ALL for your contributions.

However, yesterday, the problem resolved it self - just as mysteriously as it presented. I have no idea why...Crazy!

But thank you all again for your time and suggestions!