TB103.0b2 on Linux. I can confirm this behavior. Existing labels for a contact are not visible in Edit mode. Labels in Edit mode are just 'None'.
Bug 1778043 Comment 3 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
TB103.0b2 on Linux. I can confirm this behavior. Existing labels for a contact are not visible in Edit mode. Labels in Edit mode are just 'None'. The address book is for Google Contacts via CardDAV.