The calendar has been available for many years, though as an optional add-on.

I use Google Calendar via CalDAV. You set up a calendar in your Google account, then add a new network calendar to Lightning and connect it to your Google Calendar. This shares very easily with Android devices, since they use Google accounts and have built in apps for calendar and google mail.

Most Thunderbird users seem to prefer to use an add-on, Provider for Google Calendar to synchronize, but I see no advantage in an additional add-on when there is native support.

Other on-line calendars may work too. Lightning supports connections via CalDAV, ICS and WCAP.

If the calendar is already in Thunderbird, why do I need to get another calendar. That makes no sense. I don't want another calendar. I just want to use the calendar that's in Thunderbird and I want to synchronize it between my two computers.

Well I've told you how to use it. The rest is up to you. That's how it works. Did Thunderbird come with a ready-to-use email account? No, you had to add one yourself. Lightning isn't very different.

Lightning is a tool to implement a diary function. It comes with a default calendar which is local and so of no use to you. So you add a calendar that connects to an online resource. You can add multiple such calendars for different purposes, such as work vs personal. You can add ready-made calendars for, say, national holidays. It's flexible and customizable. But to have a calendar that is sharable between multiple computers, you need a resource that all your devices can access. That means an online calendar.

Just as your various computers share a common email store on the IMAP server. Or several servers if you have accounts with several providers.

Well that is disappointing. Since Mozilla put Sync into Firefox, I was hoping that they put that same technology into the Calendar option in Thunderbird, so that we can sync our calendars, just like we can sync our favorites. I don't need another account. I don't need another plug-in. The computer already has barely enough speed to keep it useful. I don't need another plug-in to slow me down more.

Firefox is quite trivial to synchronize. A few bookmarks, maybe a few saved tabs, perhaps a few add-ons.

Thunderbird has address books, calendar data, email account settings, email account data (i.e. message stores), often amounting to many gigabytes of data. Some of this is non-trivial to synchronize, especially if different platforms are being used. And what to do if a transplanted profile is inconsistent with server data?

But there is nothing to stop you trying to copy the profile around if that seems more sensible to you than using an online calendar. But note that you already have a calendar, and the CalDAV support is built in. No further add-ons are needed to synch a calendar.

As I said, I use IMAP to keep the mail in sync, so don't need the whole profile ... just the calendar.

First you said you need the add-on Provider for Google Calendar. Now you say you don't need another add-on. Well, which is it?

I stated: "I use Google Calendar via CalDAV". CalDAV support is built in. No add-on.

I added: "Most Thunderbird users seem to prefer to use an add-on, Provider for Google Calendar to synchronize". Provider is somewhat easier to set up but not essential. My experience was that it didn't work as well as CalDAV, but I've read people saying the opposite. YMMV.

So, you don't need this add-on. But life might be easier if you opt to use it.