The package is invalid. It may contain entries using invalid characters, as an example using '\' as a path separator is not allowed in Firefox. Try to recreate your add-on package (ZIP) and make sure all entries are using '/' as the path separator.
Le pacchetto non es valide. Illo pote continer entratas que usa characteres non valide, como exemplo '\' como separator de route, que non es permittite in Firefox. Tenta de recrear tu pacchetto de additivo (ZIP) e verifica que tote le entratas usa '/' como separator de route.