In the attached picture, clicking the sender in the Header Pane, who has already been added to an address book, shows the Edit Contact command. TB 115/W11. If you don't see it, test for a broken layout by running in safe/troubleshoot mode (hold Shift when launching TB).

Thx! I am on TB115/W11 and in both normal and safe modes I only see the edit option for a few of the emails in my address book. For all others (hundreds) I see "Add" even if they are in teh address book! Any other suggestions?

I've no idea why Edit appears for only some contacts. Check to see if you can see a difference, e.g. the non-working ones have the email address field in the wrong place, or some of the name fields are empty.