You can install this add-on. https://addons.mozilla.org/thunderbird/addon/importexporttools/

Also see https://support.mozilla.org/kb/installing-addon-thunderbird

Very many thanks for your help. Downloaded the importexport add on and set up the transfer. Used the import messages in the end. The mbox file seemed to have disappeared! Works a treat.

In reply to your private message, try via the Tools option on the Address Book's menu.

I think you'll need to select a specific address book (e.g. Personal Address Book) before the import option becomes available to you.

all sorted re address book. Thank you.