I plagiarized the question of someone else as I have the same question. I ether did something or there was an update...that changed the look of Thunderbird. I always had a column on left side of window, with all my e-mail addresses and their folders(inbox, out box, trash ect). Today it's gone and I cant find where to bring it back. I have to use GO menu option to check other addresses. They did add a menu button as described at bottom of my attachment, but this doesn't seam to be a part of the problem. I went to go, all accounts are there. There is nothing to check or indicated how to remediate the problem.