I finally got around to setting up my iCloud account on Thunderbird today, and whislt I was able to sync emails down using IMAP, my calendars were not syncing down like Outlook on Windows.
Previously in the past, I have just gone on to iCloud in a browser and then added the shared URL link into Thunderbird. This gave me visibility of my calendar but didn’t allow me to edit anything, as it is just a read-only view.
This sadly doesn’t work for me as I need to be able to edit appointments as well. My googling took me to an blog post from 2018 by Marcel Stör who has documented the steps needed to setup a read/write calendar which syncs into Thunderbird.
The steps are fairly easy to follow and surprisingly still work today.