This usually means that the address book data file is corrupt. I'd suggest you export your misbehaving address book to an LDIF file, delete the original address book then re-import its data from the LDIF file. Note that you won't be allowed to delete either ''Personal Address Book'' (PAB) nor ''Collected Addresses'' (CA) within Thunderbird. You'll have to close Thunderbird, open its profile and delete: ''abook.mab'' for PAB, or ''history.mab'' for CA. When you next run Thunderbird, it'll create a new empty address book to replace the one you deleted. Import your address to a new temporary address book, then move the Contacts (drag-and-drop) into the preferred address book.