How to sync thunderbird with icloud calendar

If you're one of those Apple users who want to use the open source Mozilla Thunderbird e-mail client, you might feel that the calendar is one thing you're missing.  There are two general methods to sync the calendar to Thunderbird.

One is something which is widely popular and involves subscribing to a read-only view the iCloud calendar and the other is a bit involved but gives you Read-Write access to the calendar. iCloud calendar supports the IETF standard known as CalDAV which is all really nice but it doesn't provide users an easy access to it's URL, which can then be added to Thunderbird.

Start off by creating a new app (or skip if you already have one) from this URL - https://appleid.apple.com/account/manage

Then click on: App-Specific Passwords

Click on app-specific passwords

Click on the "+" sign and create a name for this app so you can identify it. 

Click on the "+"

This name isn't used anywhere so you feel free to go crazy.

Name for this password (Doesn't matter what it is, it's for you to identify)

The app password is generated and keep this handy as you will need it later.

Open the Calendar app on your favorite browser: https://www.icloud.com/#calendar

From the network tab in dev-tools. If you're unable to find dev-tools, follow this guide: https://webmasters.stackexchange.com/questions/8525/how-do-i-open-the-javascript-console-in-different-browsers/77337#77337

Find Devtools from the Chrome menu on the right corner

Then, filter for collections.

Deselect your primary calendar and / or select it

Select & De-select your primary calendar a couple of times and this line will appear

Now select the URL in the network tab and extract the following colored fields "Request URL"

https://p50-calendarws.icloud.com/ca/collections/99XX99FF-X007-4XXX-9X7X-99X0X99X0000?lang=en-....blah..blah...&dsid=20001121007

Then create the URL which looks like this -

https://p50-caldav.icloud.com/20001121007/calendars/99XX99FF-X007-4XXX-9X7X-99X0X99X0000/

Open up Thunderbird and follow the instructions to add a new calendar

Select Network

Select "On the network"

In the username field enter your iCloud email address

In the URL, paste the earlier URL here and continue.

Thunderbird will now prompt for a password, enter the app-password that you created earlier.

That's it. It will work flawlessly after the initial sync completes.

Reference:

https://frightanic.com/apple-mac/thunderbird-icloud-calendar-sync/