Try this: - click 'new message' or 'write' - press F9 key ( or View>contacts sidebar) - highlight a name and click the button for To, CC, or BCC - repeat as needed - OR highlight several by using Cntl key and then press the appropriate add button.

Thanks! Your suggestions are good, but they haven't solved my problem. I can select individual names or contiguous names and get them into my To, CC, or BCC window just fine. Where I have the problem is in selecting and keeping them selected so I only have to add them to my TO, CC, or BCC window ONCE. Let's say I'm working with an address book of 500+ addresses; of those, I want to 150 to 250 for a given email. Every time I sent an email of that quantity, the recipients are different, so I can't compile a new address book. With the old Thunderbird, there were keys I could hold that would keep non-contiguous address selected until I added them to TO, CC, or BCC. So I might choose from A to H, then I to M, then N to Z. I can't figure out how to do this in the new Thunderbird in the latest MacOS. Holdling the Cntl button doesn't do it, not does holding the space bar -- both suggestions found online.

sbest64, You mentioned mac. Isn't the Cmd (command) key the one you want? I don't have any apple devices to test on, but I thought that was the key you need to start with.

Hold that, make a selection, go to the next selection, try a third selection and see if that works.

I wish that worked, but no luck. the command key only selects blocks of contiguous addresses. If I click on one address, and then click on another several addresses away while holding the command key, I will get the entire block of addresses.