I found the issue so this question can be closed :)

Once in a while I use CCleaner including the removal of Cookies. During this removal a special cookie for the EverySync Firefox add-on and this 'Enable Auto Sync' Bookmarks setting is removed. The cookie in my situation is (now) called: 5747291e-f0c1-45d6-ac6c-e377be7fe101

I have added this cookie to the section that will be ignored by CCleaner during the removal action.

Actually, I see that the Cross-site-tracking cookies are also part of the Firefox Privacy Standard settings. Anyway, we will see.

I will try and let you know when the issue is back or after a couple of weeks when the issue has been solved by switching to the standard privacy settings enabled including the social-mediatrackors.

Cookies are disabled for Cross-site- and social-mediatrackers but that is more or less the same as Firefox standard so I think to enable cookies isn't possible with Firefox. Any other suggestions?

The pPrivacy settings of Firefox you mean? And if so, are you able to specify the 'critical' privacy settings from this issue perspective so that I can look into the selected options?