Updated for Synology DSM 6.2 and Thunderbird v68.

• TbSync is the best sync extension I found, and is updated to the latest Thunderbird v68 as of 2019-09-08. Get the latest TbSync version.
• TbSync will show it's status at the bottom right corner of Thunderbird. Click on it, then add an account with your user's credentials on the Synology NAS.

• Open Calendar Synology DSM app, then hover over your desired calendar below CALENDAR on the right
• Click the down arrow and select “CalDAV Account”
• Select the line below “Thunderbird:” and copy the url. Make sure you opened the calendar through the required protocol (HTTP or HTTPS).
• Replace the NAS machine name with the fully qualified domain name, through which the NAS can be reached from the Internet.
• Every user and calendar has a specific url.

• Every user has a specific url

  domain.name.tld:<port>/addressbooks/users/bco/addressbook/

• As of 2019-09-08 connections from a client to the Synology NAS via HTTPS break. The workaround is to tunnel the connection through a Reverse Proxy. The proxy uses HTTPS protocol, the NAS is connected regular HTTP.
• Setup the proxy through DSM's Control Panel –> Application Portal –> Reverse Proxy.
• Source
  • Protocol: HTTPS
  • Hostname: *
  • Port: <port> (this port must be forwarded to your NAS in your firewall)
  • Enable HSTS
  • Enable HTTP/2
• Destination
  • Protocol: HTTP
  • Hostname: localhost
  • Port: 8008 (this is the HTTP port for CardDAV on your NAS, do not open this port in your firewall)

• Install DAVx5 from Play Store
• Setup CalDAV connection first, then CardDAV
• Login to icloud.com, select all contacts and export vCard
• Import the vCard file in DSM CardDAV Server

• TbSync
• Don't work on Synology NAS
• XML error when server sends large (response-) XML (HTTPS only) (scroll to the bottom last post)
• Workaround for Synology XML error
• How to securely manage your Synology NAS calendar
• How to Sync Synology Calendar with CalDAV Clients
• Kalender auf der DiskStation
• Ports for Synology DSM