Out of the many calendar add-ons in Thunderbird, Lightning is the best. It adds a full feature calendar system to Thunderbird and it integrates so well that you don’t even see the differences. here you can read the 14 steps needed for integrating Google calendar in Thunderbird. If you need additional screenshots visit the original website.
I copied the text for easy reference and the case that the other website got offline.
1. Open Thunderbird. Go to “Tool -> Add-ons”. Search for Lightning.
2. Install Lightning. Restart your Thunderbird.
3. You should see a new “Events and Tasks” option in the menu bar. Go to “Events and Tasks -> Calendar” and the calendar will show up in a new tab. You can start adding events and tasks to your Thunderbird.
4. Now that you have set up Calendar in Thunderbird, the next step is to integrate Google Calendar to Lightning. Provider for Google Calendar is one good add-on that can get the job done.
5. In Thunderbird, go to “Tool -> Add-ons” and search for “google calendar”. Scroll through the list to find “Provider for Google Calendar” and install it. Restart Thunderbird.
6. Next, open your browser and login to your Google Calendar account. Go to “Calendar Settings”.
7. Click on the Calendar tab, follow by the calendar that you want to integrate to Thunderbird.
8. Scroll down till you see the “Calendar Address” field. You should see three icons beside it, with the label XML, iCal, HTML respectively. Right-click at the XML icon and select “Copy Link Location”.
9. Go back to your Thunderbird. Go to “File -> New -> Calender”. On the popup window, select “On the network” and click Next.
10. In the next window, select “Google Calendar” and paste in the link that you have copied earlier.
11. Give your new calendar a name and set the color.
12. Enter your Google username and password. You might want to check the box “save password in Password manager” so it won’t prompt you for password in the future.
13. Lastly, once you see the “Your Calendar has been created” message. You are done integrating your Google calendar to Thunderbird.
14. Repeat the same steps for each and every Google Calendar that you want to add to Thunderbird. The “Provider for Google Calendar” supports a two-way sync, so you can add/edit events and tasks in Thunderbird and have them synced back to the Google server.