1. The following steps might be helpful: "Settings" > "Composition" > "Addressing" section > "When addressing messages, look for matching entries in:" > Tick "Local Address Books".

Please see the attached image.

2. The following link might be helpful: Addressing an Email https://support.mozilla.org/en-US/kb/addressing-email

Thank you. The first part of your answer helped for addressing single addresses. However, it does not work for the grouped addresses I have in separate address books (it DID work on my old computer) - the ones inside the yellow box in the attachment. I used to be able to type "Neighbors 1" and all the addresses in that book were in the To: field. Since I use these address lists every week, I need to find a way to access them; each list contains about 40 names and I can't be moving them one-by-one every week.

1. I think you should create mailing lists instead of creating address books. You can create a mailing list by clicking the button located on the right hand side of the "+ New Contact" button.

Please see the attached images.

2. The following link might be helpful: How to create and use mailing lists on Thunderbird https://support.mozilla.org/en-US/kb/how-create-and-use-mailing-lists-thunderbird

If it worked on my old computer, why doesn't it work the SAME WAY on the new one?