Firefox can’t find the file at moz-extension://a99918fb-d910-4cea-beec-73959b50bbef/popup.html

[CircleCode]

After installing this extension, a new button is added to the firefox toolbar with the slashy icon. clicking this button triggers this error:

File not found

Firefox can’t find the file at moz-extension://a99918fb-d910-4cea-beec-73959b50bbef/popup.html.

    Check the file name for capitalization or other typing errors.
    Check to see if the file was moved, renamed or deleted.

And indeed, this file does not seem to exist at all