I am using Thunderbird 52.3.2 with Lightning i.e. latest version. Have been using Lightning for years no problem. Since the end of 2017 I have been unable to snooze or dismiss NZ Public Holidays using http://apps.employment.govt.nz/ical/public-holidays-all.ics. It worked fine for Christmas but New Years day and Day after I cannot dismiss. There are other Forum and Google comments but older and they talk about unchecking the read only property. I can do this but it resets itself, I'm guessing when it refresh's currently set to every 30 minutes. Comments I have seen don't resolve the issue of the checkbox resetting. I have not made any changes to Thunderbird or Lightning except to install updates as soon as they are available. I have a Mozilla calendar and 2x Gmail calendars and they continue to work fine.