Add the Google calendar to TB: click the 3-bar menu icon, New Account, Calendar, On the Network, enter the gmail address, leave the Location box empty, since calendars will be auto detected. If asked for a password in an OAuth browser-like window, enter the gmail account password.

I followed the steps. It than has the blue button 'find calendars' when I click on that it asks for email address and Thunderbird password, I enter... press OK. .... and window closes but nothing else. (windows attached (email blanked)

If I want it to 'synch' with Google Calendar shouldn't I need to select Google somehwere ?

Your picture shows the dialog for adding a GMX calendar, not Google.

I am trying this now and still having problems. I open up TB on PC clicked the 3-bar menu icon, New Account, Calendar, On the Network, enter the gmx address, and entered password.

It asked me what I want to subsribe to ... but these were 'greyed out (see pic) Location is pre-filled and I could not remove it.

I tried 'select all' did not do anything I clicked on 'subscribe' but nothing happened.

Still Calendar invites not getting into Google Calendar

In your second picture, click in the Location box and enter caldav.gmx.com and then click Find Calendars. Enter the account password, and you should be able to subscribe calendars (see picture).

https://support.gmx.com/ios/settings/sync.html

I did what you suggested ... when I pushed'find calendars' On next screen there is nothing found, so no option to subscribe - see pic

The link you provided unfortunately refers to iPhone not PC

What happens if you click on 'Select All'?

The instructions are for iPhone, but the CalDAV location URL is the same for all devices. It worked here, as shown in my picture.

OK .. step I .. New Calendar, brings up first window, I enter email and the caldav.gmx.com I press - 'find Calendars' It brings up window wanting my gmx password (at what point do I select or enter that's it's Google Calendar I want to connect to)

I click OK, and then it brings up window where Calendaras are greyed out ..... I clicked on 'Select all' There is no chnage on screen. When I press 'Subscribe' window closes, nothing happens

If you want to connect to Google Calendar, why are you trying to add a GMX calendar? They are two different providers. Enter your gmail address in the User name box to add Google Calendars (leave the Location box as is).

Because I access my Google Calendar using the same email as my GMX email. I have a gmail account, but don't use it, I use my email account for Google apps.

I'll see if I can find my Android gmail account.

OK update ... progress - I did that using gmail, it found a Calendar and it brought in the events from Google Calendar into Firefox However if I create an appointment in Firefox Calendar it does not appear in Google Calendar.

If you login to a Google calendar with a gmx account, try adding it by entering the gmx address as the User name and for the Location use:

https://apidata.googleusercontent.com/caldav/v2/yourname@gmail.com/events/

OK .. think this was a big part of the way forward It found calendar, asked for my google pwd for my gmx account, and all proceeded with no errors. However if I create entries in Google Calendar they do not appear in Thunderbird, and vice versa.

Note that the default refresh interval is 60 min., which can be changed in calendar Properties (context menu), or manually (Synchronize). My Google calendar syncs both ways, but the User name is the gmail address.