I did locate this: https://github.com/thunderbird/import-export-tools-ng/issues/587 It would have been helpful if they had put a link to the bug report. However, it looks like this issue was reported some time ago as I also found this: https://github.com/thunderbird/import-export-tools-ng/issues/284 Not sure whether this is something to do with how the addon was installed...but would you test something for me. Can you perform a reinstall but using a specific method. 'Tools' > Addons and Themes > Extensions Select 'ImportExporttools NG' and click on the 3 dot icon - select 'Remove' Exit Thunderbird completely and wait a few seconds before restarting again. This should have completely removed the addon extension and updated all files, but I need you to check it has been removed - no longer visiable in any menus nor in Addons and Themes etc. Download the addon from this link so you download a .xpi file. https://addons.thunderbird.net/en-us/thunderbird/addon/importexporttools-ng/ In Thunderbird * 'Tools' > 'Addons and Themes' > 'Extensions' * Click on gear wheel icon and select 'Install addon from file' * Locate the xpi file you downloaded and click on Open It should then auto install then exit Thunderbird completely - wait - then restart Report on what you see in the Menu.