If you are a Gmail user chance is that you are also subscribed to other Google services such as Google Calendar. We all know that Thunderbird users can integrate Gmail into the desktop email client to archive messages, access them while they are offline and manage the account almost as if they would use the Google webmail interface.
There has not however been a way to integrate other Google services that are related to Gmail into Thunderbird. The Google Calendar Tab add-on for Thunderbird 3 changes this. It basically only displays the web interface of Google Calendar in a new tab in Thunderbird.
And since the extension uses the same account all information from that Google Calendar account are already available in Thunderbird. It is possible to view, edit and add events to Google Calendar which are then also visible on the Google Calendar website. If the calendar has been configured to show popups then they will be shown in Thunderbird as well.
The extension for the email client supports both Google Calendar and Calendar for Google Apps. Google Calendar Tab is an interesting add-on for Thunderbird that aids Google Calendar users who work with Thunderbird to integrate their calendar into the email client. There are other options to do that. IIRC it is possible to integrate Google Calendar into Lightning as well.
Related Articles:
Sync Google Calendar With Thunderbird LightningAdd a Calendar to Thunderbird
A Closer Look at the Lightning Calendar Add-On For Thunderbird
Display Google Calendar in the Sidebar
Redesign your Google Calendar with a custom skin
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Why not just use the Lightning extension along with the Provider extension for seamless integration?
https://addons.mozilla.org/en-US/thunderbird/addon/4631