You have opened the "Chat" tab by inadvertance. Hit the keyboard shortcut ⌘ + 1 to open the Mail tab with your accounts and folders/messages

I haven't figured yet what's the right way to fix this problem, but I think it's caused by a corrupt preferences file issue that appears to happen only on Mac OS. The following article explains what's a profile and how to locate your profile folder: https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data # Quit Thunderbird. # In the Finder, go to <b>~/Library/Thunderbird/Profiles/</b>. Make a backup copy of that folder, just in case. # Try to figure which is your main profile if you have more than one profile folder there. You may be able to figure that by looking at the folder contents and the modification dates of the files. In particular, the <b>Mail</b> and/or <b>ImapMail</b> folders within the profile folder should contain folders named after the incoming server of each of your accounts. # Pay attention to the <b>prefs.js</b> file. There should be only one such file. Do you also see additional <b>prefs-<i>N</i>.js</b> versions of that file? # Rename <b>prefs.js</b> to something else, e.g. <b>prefs.bak</b>. # If there are additional <b>prefs-<i>N</i>.js</b> versions of that file, pick the most recent (presumably the highest numbered) one and rename it to <b>prefs.js</b>. Otherwise, we'll just let Thunderbird create a new <b>prefs.js</b>. # Launch Thunderbird. You may have to set up your mail accounts again, in which case we may need to reconnect them to the folders Thunderbird was using before to store mail for them. Post back with the results.