A new version of ThunderSync is available at https://addons.mozilla.org/de/thunderbird/addon/thundersync/versions/. Seems it will take a couple of days until version 1.2 gets reviewed by mozilla.org.
Users can now enable ThunderSync to compare contacts at Thunderbird startup. Contacts are also automatically compared when the ThunderSync dialog is opened.
The comparison algorithm was revised and support for an unique ID (UID) was added. Identification and mapping of external and internal contacts is now done via UID, so that other contact properties are changeable (new name, other address).
If no UID is given, internal and external contacts are mapped by percentage of matching properties and an UID is assigned afterwards.
Finally, the UID is used for file name creation. This avoids problems with empty contact names.