I have a Thunderbird address book on an old PC. I also have a new Thunderbird address book on a new PC, containing adresses which I have accumulated since using the old PC. I wish to import the data in the address book on the old PC into the address book on the new PC. BUT I have not been able to find out whether importing the old data will SAFELY MERGE & APPEND the old data with the new data in the new adress book, or whether importing the old data (e.g. via a .csv file?) will REPLACE the data in my new address book (and thereby overwrite and LOSE the new data which I have accumulated). So, how can I safely import the old addresses without losing the new addresses? This is a situation akin to synchronising 'favourites' etcetera in the Firefox browser - Thunderbird users could do with a similar feature to enable then to synchronise their Thunderbird address books across all of their devices running Thunderbird.