Cert Expiry од Will Sparks

CertExpiry is a user-friendly Firefox extension designed to keep you informed about the validity of website certificates. It constantly checks the number of days left until the certificate expires and displays it on a padlock icon in the toolbar.
When there are less than 29 days left, the icon turns red, serving as a visual reminder to take action. In cases where the site connection is not encrypted, it shows an unlocked red padlock, highlighting potential security risks.