How to merge multiple mailing lists in Thunderbird

Hello Vika,

Easy!

In the list you want to move to another, select all contacts and drag-and-drop the whole set into the other list.

To select all contacts at once, select one of them, any, and press Ctrl A (A for All). All contacts will be selected in blue.

Then click in the blue selected area and, without releasing the left mouse button, go to the destination list on the left. Release the click when it is selected in blue.

The contacts will be duplicated into the other list.

All that remains is to delete the original list (right-click / delete).