While searching for an extension or utility to manage vCard files and/or attachments in Thunderbird, I stumbled across the Kaosmos website.
This site is a treasure trove of extensions for both Thunderbird and Firefox which are real time savers. I installed 3 straight from the get go including MoreFunctionsForAddressBook which does exactly what I want (i.e. import and export vCards to the address book) and is absolutely brilliant. And the extensions are updated regularly which is a bonus.
I just don’t understand why the author hasn’t put these extensions in the Mozilla Add-Ons repository as there must be thousands of other users who could use a number of them.