Have you asked apple why some contacts in icloud don't sync with their operating system contact manager? While it may be something odd in the way Thunderbird or the addon implements caldav that is only affects the desktop application would indicate that the problem is there. It would also be super handy if the well resourced Apple folk told you, and us via you, what the problem was with the Thunderbird contacts so a bug could be filed for Thunderbird if that was the appropriate action.

When I googled this I kept getting references to something called groups/labels and it is quite probable that Thunderbird does not use one of the groups you have subscribed in your contact app. Perhaps not even allocating one as Thunderbird has address books, not contact groups. I notice that the specification has there proprietary fields for company names, and knowing how apple like to get their name out there, it may be the desktop is looking for some apple code in the type Somewhere like the namespace which is normally blank https://datatracker.ietf.org/doc/html/rfc6350#section-10.2.3

Based on this https://github.com/jobisoft/DAV-4-TbSync/issues/299 it would appear that all syncing is actually done by Thunderbird natively, and not by the addon so this is not a case of you are using an addon, so it is a problem with the addon.

Thanks Matt Yes, I had a long conversation with Apple Support. Because there was no obvious answer, we tried setting up a new User to see if that overcame the problem - but with the same result. I requested that the problem be submitted to Apple Technical Support and he assured me that that would be done. Whilst we were screen sharing he was also researching his data base but with no answers. He stated that in his quick search of Mozilla/Thunderbird it appeared that Thunderbird may have some different 'format' options for the entries (my non-technical words) and that a different format might solve the problem. I could not see how to access these options. This is similar to your comments regarding proprietary fields. However the new address entries I created in Thunderbird were only test entries with a name - all other fields were usually left blank. This does not necessarily mean that the Apple Desktop app was being confused by the Thunderbird field versions even though they were blank. I do not have groups in any of my Address/Contacts apps so that will not be a cause. Many thanks for your response.