This is a known bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1662285

Workaround: hold Ctrl while dragging and dropping.

Thank you for this. Although I can now "drop" an address into the mailing list, it dose not appear when I look at the mailing list afterwards. Thanks for the help but I am still struggling!

Is the mail address for the contact in another box besides the Email field in the contact Properties? This sometimes happens when a contact list is imported without matching the input fields to the Address Book fields.

Looks possible. I had to import from Windows 10 Mail via .CSV (not pretty!) looks like I have to juggle the fields to get this to work, I am starting to get things working, thanks for the help.

The best way to import a csv is to first edit it in a spreadsheet or csv editor and organize and rename the columns to match the TB structure.

http://kb.mozillazine.org/Import_address_list_from_text_file

Thanks for your patience and help. :-)

I have been having identical problems to DougSteptoe. I think I have solved it but it is messy: I can drag addresses into an empty existing address book (say Personal Address Book), then go to this list and select all and move it (with Ctrl held down) to the new list. It works and for sure beats typing in each entry to the new list. Thank you for the Ctrl key tip Tony Cottrell

This is a known bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1662285 Workaround: hold Ctrl while dragging and dropping.

This workround also seems to have stopped working now :-(

It works here in 78.3.1. Create a new test address book with a contact and mailing list and see if the contact can be added to the list by drag and drop + Ctrl.

Now this seems to have been addressed, a new issue has emerged - as I include addresses in groups, they become multiplied in the main address book