How do I get Thunderbird to open an ICS attachment directly in Lightning's calendar?

For Thunderbird 15.0 / Lightning 1.7 the following procedure works for me:

1. Open the "Today Pane" ie. the Events panel on the right hand side next to the email list.

2. Drag&Drop the ics-file onto that pane.

In my case (Kubuntu 12.04) it will create the event. Hope this helps.

N.B In Windows 7, you need to hold the CTRL key down while dragging the ICS file on to the events pane.

Right click on the mail and click Convert > Event

On Windows:

1. Save the attachment to the desktop
2. From Events & Tasks select Import
3. Select the .ICS to be added to the Calendar
4. From the calendar pop-up select which calendar to add the event