OUverture pages Mozilla impossible

Bonjour,

Depuis ce matin je ne peux ouvrir de pages avec Mozilla
j'ai le message d'erreur suivant :
This Connection is Untrusted

You have asked Firefox to connect securely to login.yahoo.com, but we can't confirm that your connection is secure.

Normally, when you try to connect securely, sites will present trusted identification to prove that you are going to the right place. However, this site's identity can't be verified.
What Should I Do?

If you usually connect to this site without problems, this error could mean that someone is trying to impersonate the site, and you shouldn't continue.

This site uses HTTP Strict Transport Security (HSTS) to specify that Firefox only connect to it securely. As a result, it is not possible to add an exception for this certificate.

login.yahoo.com uses an invalid security certificate.

The certificate is not trusted because the issuer certificate is unknown.

(Error code: sec_error_unknown_issuer)


Quelqu'un peut il m'aider svp.
Merci