Firefox Calendar would essentially be a calendar that other addons can interact with. In the booming digital age, everything is done online, so why not have a calendar that any online services can automatically access (via addons). Addons made for specific website (or others) can add events and modify/remove only their events.
Any online webpage with an addon would be able to sync a user approved event to the users calendar. i.e. As you hit the 'success' landing page after making or scheduling an online purchase, an addon would grab the anticpated dates, create an event and ask the user to add to their calendar.
Additional methods includes having a button loaded into a webpage to autamatically grab dates and create new events. Mozilla can maybe add snippets so that addon creators can quickly implement as well as avoid duplicate event creations.
Not all event trackers has to be 3rd Party, but some event tracking can be native (by firefox).
Native Implementation would have the best chance of success given the range of utilization possible with a unified calendar. There is just so much that can be integrated into one Unified calendar. Notifications can be up to the enduser. When cloud storage is finally implemented, it can be accessed across multiple devices, seemlesely.