Cert Monitor от Elena Cotton

CertMonitor is a practical Firefox extension that enables you to monitor website certificate validity with ease. It quickly checks the number of days left until the certificate expires and displays the count on a padlock icon in the toolbar.

When there are less than 29 days remaining, the icon turns red, prompting you to take appropriate measures. Furthermore, if the site connection is not encrypted, it shows an unlocked red padlock, indicating potential security issues.