Obviously, you need to install the Thunderbird Lightning extension. You can then create a new network calendar:

Choose CalDav format and specify location (adjust port to your settings):

Replace mail@company.com with actual user email address.

Note for iCal users: use the following URL instead:

Choose a name, color and set the associated email address (as set in mail setup)

Provide your credentials, login is Active Directory account name, i.e. domain\account

Finish:

You may want to enable the (experimental) Lightning calendar cach feature to avoid the possibly long calendar load task delay on Thunderbird startup: double click on the calendar in Lightning left pane and activate cache. Restart Thunderbird.

Proceeed to Thunderbird directory setup