We can use Google calendars in two ways. Either have read-only access to public calendars or  use a bi-directional calendar, with both read and write-access.

1) Read access

If you only need read-access to a Google calendar, you can use the link of the public ICS file which is provided by Google, as indicated on this Google Help article: View from other applications

2) Read and write access

Downloading Provider for Google Calendar

If you need both read and write access to your calendar, install the Provider for Google Calendar from addons.mozilla.org.

1. Open the Provider for Google Calendar addon website.
2. Download the file by right-clicking on the Download now link and choosing Save as.

Installing and configuring the add-on

1. To install the add-on, click the menu button  or select the Tools menu from the Thunderbird menu bar and choose Add-ons. The Add-ons Manager page will open.
2. Click  on the Add-ons Manager page and then select Install Add-on From File….
3. Locate the downloaded file on your machine and click Open.
4. You will be prompted to install. Click Install Now to complete the installation.

Using above methods we can access google calendar in Mozilla Thunderbird Email