Sync with Thunderbird and Android - CardDAV & CalDAV

Updated for Synology DSM 6.2 and Thunderbird v68.

TbSync

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)