Zerknij na to. Nie testowałem, bo nie używam Thunderbirda, ani zadań Synology.
Create a new calendar in Tbird>On the Network>Next>CalDAV
Set location as...
NOTE: This is the same URL format as a regular CalDAV calendar link from Synology Calendar, with the exception of "home_todo" for "home"
Name your task calendar (I used "To Do") and allow Notification and Offline access.
You must leave this "calendar" checked for the to do entries to display in Tbird's task window. You can add/edit/delete tasks and sync in either direction. Tasks will not appear in Tbird's calendar calendar... only the Task window.
IMPORTANT: This syncs with Synology's Inbox task list. If you create a new task list in Synology Calendar, you will need to create a separate TBird Calendar... the CalDAV link for the new task list can be constructed by viewing your browser's URL when viewing each of Synology's Task list...
Look for "todo_plugin@by_list@/<username?/XXXXXX/" at the end of the browser URL, and insert the six characters "XXXXXX" in place of the "home_todo" in the link you used for the Inbox list. Such as: