If you have the Lightning calendar extension installed on Thunderbird and have the Events pane open.
You can add an ICS file by holding the CTRL key and then dragging the ICS file on to the events pane. (The Control key needs to be pressed on Windows, but apparently it may work without pressing the CTRL key on Linux)
The only problem with it seems to be it doesn't manage to parse the start time and end time from the ICS file properly, it just marks the event as an all day event and disregards the actual start and end times (on windows anyway, not sure about linux) the rest of the info seems to be picked up correctly.
BTW There is a bug open in Bugzilla for this issue (ICS files not being able to be directly opened in Thunderbird/Lightning). If enough people vote for this bug, its more likely it will get fixed. To vote for the bug to get fixed, create an account on Bugzilla, then go here: https://bugzilla.mozilla.org/show_bug.cgi?id=357480& and under the header titled "Importance" there is a link that says "Vote" click that then add a tick to the bug and then click "Change Votes".