Thunderbird has always had read only access to the mac address book. I am guesing this is what you are seeing, rather than problems with the "thunderbird" personal address book or the collected addresses books.

I do not use the mac address book - it's the Thunderbird Address book that I'm having trouble with

and I used to be able to create lists and add emails to them, I want to know why this has stopped.

Have you tried this: If you add the contact to the 'Personal Address Book' and then left click hold down on the added contact to grab and drag over to hover over the Mailing List name - as shown in image you supplied - then release to drop into the Mailing List, does this add contact to that Mailing List ?

At link see option 2 - there is an image explaining. https://support.mozilla.org/en-US/kb/how-create-and-use-mailing-lists-thunderbird#w_create-a-mailing-list

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