Using this method to add a Google calendar solved the problem for me:
http://blog.mozilla.org/calendar/2013/09/google-is-changing-the-location-url-of-their-caldav-calendars/

ThunderBird 24, Lightning 2.6.1 provider for Google Calendar 0.25
Using this method to add a Google calendar solved the problem for me:
http:// blog.mozilla. org/calendar/ 2013/09/ google- is-changing- the-location- url-of- their-caldav- calendars/ 

ThunderBird 24, Lightning 2.6.1 provider for Google Calendar 0.25