Hello,

The plugin probably placed itself in the Firefox application directory. Please see Thunderbird add-ons - frequently asked questions for the steps you can take to remove it.

Let us know if that works!

Extensions that do not have a "Remove" button are installed by other software and are not under control of the Firefox extension manager. Such globally installed extensions are usually installed in a location that Firefox scans for installed extensions.

• https://developer.mozilla.org/Installing_extensions

Extensions installed this way need to be removed via the settings (options/preferences) of the program that has added this extension or this program has to be uninstalled. In Firefox you an only disable such an extension.