Simple & perfect Rated 5 out of 5 stars

Perfect add-on if you you wish to block ALL cookies, except for websites you choose to trust. And even then you can choose to permanently allow the cookie, or just for this session (it will be automatically deleted when you close FF if I'm not mistaken).
Glad to have found a version that my FF12 likes, at https://github.com/vrillusions/permit-cookies/downloads

The icon for this add-on sits in the add-on bar right after installing, without the need for an extra add-on. I´m very happy with this add-on!