Did you try to disable the two Facebook extensions that The System Details list shows?

Are there any messages in the Web Console about blocked content?

• "3-bar" menu button or Tools -> Web Developer
• https://developer.mozilla.org/en-US/Tools/Web_Console

If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block content (Adblock Plus, NoScript, Disconnect, Ghostery, Privacy Badger, uBlock Origin) always make sure such extensions do not block content.

• make sure your extensions and filters are updated to the latest version
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Firefox shows the shield icon at the left end of the location/address bar in case Content Blocking is blocking content.

• click the shield icon for more details and possibly disable the protection

You can check the Web Console about what content is blocked

• "3-bar" menu button or Tools -> Web Developer
• https://support.mozilla.org/en-US/kb/tracking-protection
• https://support.mozilla.org/en-US/kb/content-blocking

If you use Adblock Plus then you can consider uBlock Origin as a replacement to see if that works better (lower memory footprint).

• https://addons.mozilla.org/firefox/addon/ublock-origin/

You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History -> Show All History" or "View -> Sidebar -> History").

Using "Forget About This Site" will remove all data stored in Firefox from this domain like history and cookies and passwords and exceptions and cache, so be cautious. If you have a password or other data from that domain that you do not want to lose then make sure to backup this data or make a note.

You can't recover from this 'forget' unless you have a backup of involved files.

If you revisit a 'forgotten' website then data from that website will be saved once again.

• https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox

I disabled all add ons Restarted firefox in safe mode disabled anti virus (avast) tried a few things on your list, but many were a bit above me so eventually I capitulated and reset firefox. Mystery remains unsolved, but problem sorted by reset. Now to re-customise - one by one ;-)

There were a few red lines in the web console (could not deal with that)

Only one there now "Failed to get service worker registration(s): Storage access is restricted in this context due to user settings or private browsing mode".

Thanks for your help.

"Failed to get service worker registration(s): Storage access is restricted in this context due to user settings or private browsing mode".

Service workers are usually used for showing notification. You can create a cookie allow exception in case you delete cookies when you close Firefox if you want to see these notification.

• Options/Preferences -> Privacy & Security -> Cookies and Site Data -> Manage Data