I am using thunderbird email with their calendar from file>new calendar. I have google calendar on my phone and want to sync both. I cannot download Provider for Google Calendar add-on via the link on firefox/thunderbird web pages. I get the error, "This file could not be installedbecause it has not been verified" when trying to install the package from: https://addons.thunderbird.net/en-US/thunderbird/addon/provider-for-google-calendar/ Do I need to take it somewhere to get the verification???