If you have lightning enabled and try to turn off some of the other addons you'll see which addon is conflicting. Lightning is tightly integrated with Thunderbird, allowing it to. Allows managing calendars locally or subscribing to network calendars. Basically, Thunderbird was an attempt to represent the patterns of activity of a powerful, mysterious force in such a way that could be grasped and if not totally understood. Currently the only way to get desktop offline access to your tasks from Toodledo is to use the Chromatic Dragon sync tool, and of course that only works if you use Outlook.
Note that Lightning does not just add a calendar to Thunderbird, but also a task manager.
