What are the cookie settings? Outils > Options > Vie privée > Historique > Règles de conservation: "utiliser les paramètres personnalisés pour l'historique" --- See also Cookies - Information that websites store on your computer

Thanks, I tried to set cookies to "Ask each time" but it is hard to surf with this and still the same message : "Authentification impossible car votre navigateur n'est pas configuré pour accepter les cookies de session." The site works with Internet Explorer 6.

Try the default setting to accept all cookies. Tools > Options > Privacy > Cookies: [[X] "Accept cookies from sites"

You may need to remove existing cookies from that website.

Here are the settings that were already used. Even removing cookies from this website do not change anything.

I will try going back to a previous Firefox version.

Did you check the Cookie exceptions?

You can inspect the permissions (exceptions) for the currently displayed website in "Tools > Page Info > Permissions".

A quick way to see the cookies from the website in the current tab is to click the favicon at the left side of the location bar A click on the "More Information" button will open the Security tab in "Tools > Page Info".

You can inspect the cookies if you click the "View Cookies" button on the Security tab in "Tools > Page Info". Clear the search bar in the Cookie Manager to see all the cookies.