My work around. If there are only one or two to add: type in first few letters and it will fill other fields. If you have a lot of addresses to add, open the Address book, click to forward the email, minimize the Thunderbird program, change the size of the forwarding email and the address book so that they both are visible on the screen.
Click and drag from the address book to the "BCC" bar.
If you are sending am email to a lot of addresses you should be using Bcc.