Use Google Calendar with Thunderbird 38.1

• download and install extension Provider for Google Calendar
• by locate to Tools menu (Show menu bar by press "Alt" button) and click Add-ons at left side click Extension

• at Gear menu select Install Add-on From file... and browse to dowloaded file

• if install it successed will show follow this (Lightning install by default installer Thunderbird 38.1)

• then add account Gmail by near minimize button click "switch to calendar tab"

• right click space area bottom of list calendar and click "New Calendar..."

• Select "On the Network"

• if install Provider for Google Calendar successed will show Google Calendar choice and select it

• then fill your account gmail and select IMAP (i can't use POP)
• step it about this

Ref

• https://support.mozilla.org/en-US/kb/using-lightning-google-calendar