Open up your calendars tab in Thunderbird. Right click on the calendar you wish to share.

Select the option that says "Sharing..."

Click the button in the upper left-hand corner of the window that appears.

This should bring up a window that says "Subscription...". Here you can add a contact to share your calendar with. Enter the contact's email address in the top bar and click add.

Double-click the new contact you intend to share your calendar(s) with.

Here you can adjust the permissions your new subscriber will have. Your settings will vary depending on what events you would like your subscriber to view and edit/ create. Press Update when you have finished adjusting your settings.