I am sorry but there is a persitent cookie linked to this addon.

When i start a session and i go directly in my setting i fond a youtube.com cookie.

My FF clears cookies at the end of the session so there should no cookie at all at the beginning of a session.

I couldnt find info about that youtube.com addon cookie on the developer website.

Thanks for explaining why this cookie is created when the browser loads all the addon and when i start to use youtube.