It's built-in, no add-on needed.

Ok, i found a bug, i think, where can i post info about it?

You may want to start here explaining what the problem is.

Please explain in detail

• what you're doing,
• what happens,
• what you expect to happen.

I did it here, is it acceptable? https://forums.mozillazine.org/viewtopic.php?p=14977155#p14977155

When your contacts are in a LDAP directory I'd suggest you set up a LDAP address book, not CardDAV.

It's indeed carddav. https://kb.synology.com/en-us/DSM/tutorial/How_to_use_CardDAV_to_sync_Synology_Contacts

It can be switched, but it's no matter where users are located, contacts anyway using carddav.

https://kb.synology.com/en-us/DSM/help/Contacts/contacts_admin_settings?version=7

So where can i post a bug info ?

For CardDAV you need a user name and the correct URL for your Synology Contacts. I have no idea about Synology, so I'm afraid I can't help with this. Also, I can't tell whether this a TB or Synology problem.

You may want to raise a bug at Bugzilla, but don't hold your breath. https://bugzilla.mozilla.org/

Understanding Mozilla: Bugzilla https://gerv.makes.org/popcorn/1bn6

https://bugzilla.mozilla.org/show_bug.cgi?id=1895601 and i also added 'error console' output