Hello there joemac8. We have read your message. We try to help you. https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data https://support.mozilla.org/en-US/kb/moving-thunderbird-data-to-a-new-computer Here is the location: C:\Users\<Windows user name>\AppData\Roaming\Thunderbird\Profiles\<Profile name>\ 'profile name' folder is usually named ''''xxxxxxxx.default' folder, '''where the x's are letters and numbers. Image below shows location of .mab files. abook.mab is the default created 'Personal Address Book' history.mab is default created 'Collected Addresses' other *.mab files are ones you created. You may need to make hidden files and folders visible. http://kb.mozillazine.org/Show_hidden_files_and_folders '''For Mac to go to your adresboek:''' File -> Use Mac Catalina Address Book But if there is a security setting on Catalina that has to be set to allow the address book to be used. check this as well then System Preferences > Security & Privacy > Privacy You'll see a few Apps and System Functions where you can allow other apps to access them. Just copy and paste. https://support.mozilla.org/en-US/kb/configuring-lightning Greetings Firefox volunteer.