Which button, and where?

The one in the Mail Toolbar. Thanks in advance for the help. Phil B.

Could we see a screenshot of the empty address book, please?

https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem

Is there an explicit prompt to start a new address book, or merely the opportunity to do so?

Hi Zenos, Many thanks for your help.

I attach the screenshot of the 'address book' page that appears on clicking through from the Mail Toolbar.

yours Phil B

Please post image showing the .mab files in your Profile folder.

In Thunderbird Help > Troublshooting Inofrmation click on 'Show folder ' button

a new window opens showing contents of your profile folder. Sort by 'Type' to get all the .MAB files together Post image which shows all of the .MAB files.

Hi Toad-Hall Herewith the Mab Files screenshot.

Hope it helps to sort things out.

Thanks for the help, yours Phil B

Thanks to those who started to offer help. Sorry that you were unable to complete the task. I have found out what it was! An add-on, which I think was: "Theme Font & Size Changer 49.6", when disabled allowed me to see the address books again.

That add-on has given some trouble recently. This is a new (to me, anyway) problematic side effect of using it.

If you still need the ability to change the size of text in the UI, you can do so using a file.

If you don't already have one, create a folder named chrome in your profile.

If in that folder, you don't already have a userChrome.css file, create it.

Add the following, as necessary, to that file:

/*
 * Do not remove the @namespace line -- it's required for correct functioning
 */
@namespace url("http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"); /* set default namespace to XUL */

/*
 * Make all the default font sizes 10 pt:
 */
* { 
    font-size: 10pt !important; 
 }

Change the 10pt to whatever meets your needs.

Thanks for that. I am not sure what the add-on was doing, and I shall chicken out and just leave the default settings.