Hi, the webextensions api doesn’t support the direct manipulation of favicons. Sometimes corrupt/outdated favicons persist... You can delete favicons.sqlite in your profile folder and run the extension again. The location depends on your operating system: http://kb.mozillazine.org/Profile_folder_-_Firefox#Navigating_to_the_profile_folder

If you click the question mark next to the play button you will find some explanations.