Following this tutorial, you can use Thunderbird to manage your iCloud calendar. At one point, you will need to copy an url and extract some informations from it. You need to do that by hand, which is error prone.
This tool eleminates that. Paste the url in the first input field, and it will generate the CalDAV url in the second input field for you, which you then copy and paste into Thunderbird.
Nothing is transmitted to any server. All is done locally in your browser.