First line: thanks to bordy for piecing this together.
Now on to the business at hand. Who uses Thunderbird? Ok, a large number of you. Who uses a calendaring system? Whoa, a lot of you ehh. Well how would you like to tie Thunderbird into Google Calendar with read and write access? Cool, keep reading. You’ll be setup in five minutes. It’s really pretty painless.
Getting the Plugins
Download the Lightning Addon for Mozilla Thunderbird
Download the Google Provider Addon for Mozilla Thunderbird (requires Thunderbird 2.x, see here)
Installing the Plugins
Open Thunderbird, navigate to “Tools” and select “Addons”. Drag-and-Drop the two files you just downloaded into the box. They will ask for installation confirmation, select Yes. When these are done you’ll need to restart Thunderbird. You will be prompted with a window to do so.
Visit Google Calendar
Before we can configure your new calendar in Thunderbird we’ll need to get just a bit of data from your Google Calendar account. Login to Google Calendar (I’m assuming you already have an account, if not you’ll want to set one up.. but that is outside the context of this tutorial.)
Navigate your way to “Settings” (top right).
Navigate to “Calendars” in the Calendar Settings area.
Navigate to the calendar you want to share (as you can have multiple calendars, we need to do one at a time)
Navigate to the “Private Address” and select the XML link via right-click. Select “Copy Link Location” in your Firefox browser.
Configure Lightning
Now that you have the link to your private Google Calendar address we can configure Lightning to use it. Go back to Thunderbird now and select “New” in the Calendar tab of Lightning (bottom right, third tab).
Select “On the Network” for the location of your calendar system.
Select “Google Calendar” on the next prompt. If you don’t see a Google Calendar option you’ll want to verify that the Google Provider Addon was installed properly.
Paste the link we copied from Google Calendar (the XML we copied just in the section previous) into the “Location” box.
The next step will ask for a calendar name and color. I’ll leave that part up to you. 
The last step is providing Lightning / Thunderbird with your Google Calendar username and password for Google Calendar. The username should have been auto-harvested from the link you pasted, but the password will be needed.
Using Your Calendar
At this point you should be able to create appointments within Lightning and they’ll very quickly propogate to Google Calendar. To get started select the Calendar tab within Lightning, right-click on a date and time and select New. I’m sure you can take it from here.. Enjoy.
(some of these steps taken from here.)
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