Hello For information purposes. Go to your profil folder https://support.mozilla.org/en-US/kb/profiles-where-thunderbird-stores-user-data#w_how-do-i-find-my-profile Try, session.json Stop Thunderbird Delete, or, rename the '''session.json''' file (session.json-old for example) Start Thunderbird Perform your test Stop Thunderbird A new session.json file will be created Start Thunderbird And see For information purposes. Thunderbird Add-on Database Analysis https://thunderbird.github.io/webext-reports https://thunderbird.github.io/webext-reports/atn-tb128.html If you wish to test, Start with Inbox. https://addons.thunderbird.net/thunderbird/addon/start-with-inbox Installing an Add-on in Thunderbird. https://support.mozilla.org/en-US/kb/installing-addon-thunderbird Enter the add-on's name '''Start with Inbox''' or enough of it to limit the search results returned to a manageable number and press Enter or click the magnifying glass icon.