I ran a search on DDG for “apple calendar on thunderbird” and found these search results.

It’s a complicated process to get access to your iCloud Calendar. Read the comments on each link if you run into errors. There’s the plugin way using Mozilla’s Lightning Calendar with additional add-ons, the hard way using the web browser inspector, and a Ruby script that will provide all the links. For each, you will need to use an app-specific password. I explained how to get that in my Sync iCloud Contacts with Mozilla Thunderbird. Instead of redoing the work, I’ll provide the links.

Using Mozilla Thunderbird add-ons: https://www.systutorials.com/synchronizing-thunderbird-calendar-with-apple-icloud-calendar/

Finding the calendar link using the web browser’s inspector: https://frightanic.com/apple-mac/thunderbird-icloud-calendar-sync/

Running a Ruby script to display your calendar links: https://github.com/midnightmonster/icloud-calendar-urls