About this Add-on
Be advised that as of Thunderbird 78 (Add-on version 6.0), the add-on no longer changes the original "Get messages" button. Instead, the addon now has a completely independent button that you have to drag from the "Customize" dialog into the toolbar.
The source code of the add-on is open source and is available at GitLab.
You are welcome to submit your feedback in English and German in the forum. We can also best solve any problems there. You are also welcome to contribute to improvements for the translation of the add-on there: https://www.thunderbird-mail.de/forum/board/85-abrufen-senden-schaltfl%C3%A4che/