Calendar 1.0 rc 2 now comes with offline support packaged for CALDAV and ICS calendar providers. For CALDAV calendar it allows retrying of events if for any reason, the updates failed to be sent to the server.
You can find it at its new location on the addons.mozilla.org beta channel (If you still get 1.0rc1, then please try again in a few hours when the page cache has been updated).
If you encounter any issues, please file a bug at bugzilla.mozilla.org.
Or you can share your experiences via comments below.
Happy to hear from you regarding your experiences. Hope you enjoy the feature.