Well, I discovered on Monday (November 17th) Google changed the calendar sign-in API. This now breaks the older version of the Provider for Google Calendar add-on. However, I was able to get all my calendars to work (for the most part) with Thunderbird/Lightning now. I linked/subscribed my school and US Holiday calendars to my main GMail account.
The setup process is much simpler now with the newer version of Provider for Google Calendar. You no longer need to supply your calendar’s public/private XML address. Instead, you will be prompted to enter (or choose) the Gmail address associated with your calendar. The next step will allow you to choose any of the calendars you have linked/subscribed to your main GMail account.
One thing to keep in mind though, the additional calendars you choose are going to be read-only. In order to make changes to my school calendar (which is something I don’t do that often) I need to sign into my school’s GMail account then go into the calendar. The only way to edit this calendar in Thunderbird would be to add it directly. However, I tried this and when I attempted to sign-in and clicked the button to submit, I get a ‘Connection Timed Out Message’. I am guessing this has to do with the limited web browser abilities in Thunderbird.