Olaf Deterding-Meyer

toolbar.customization.usesheet is set to false - but: I checked behaviour again, also playing around with other extensions, and have to admit: there seems to be a general issue with Thunderbird's main toolbar, not explicitly with Mail Redirect. So I interpreted the error mesage in a wrong way, and fixing it will not solve my toolbar issues. But be aware that your attempt to copy the icon to the installation directy will not work in Windows 10, for you need admin rights to write data in sub folders...

Toolbar icons of all custom extensions invisible