Can't copy (can only move) a contact from one address book to another

I have multiple address books in Thunderbird. For example, I have one address book for families in our small school, and within that address I have a mailing list for elementary and another for secondary. I have a separate address book for the parents in my math classes and within that address book I have a mailing list for each class. Until the latest update, I could copy a contact from the mailing list in one address to a mailing list in another, thus copying the contact into the corresponding address book. Now I can't do that. I can copy within an address book to different lists, but if I try and drag a contact, I can only drag it to another address book, which actually moves the contact, deleting it from where it came from. Why was that changed in the update and what do I do now? I want to keep the address books separate!