Certificate expiration is a Firefox extension designed to enhance user awareness regarding website security by monitoring SSL certificate expiration dates. This tool provides a straightforward interface that indicates how many days remain until the certificate for the currently viewed page expires. The countdown is conveniently displayed as a badge on a padlock icon located in the Firefox toolbar, allowing users to quickly assess the security status of the site they are visiting.

Additionally, the extension employs a color-coded system to alert users: if a certificate has fewer than 29 days remaining, the padlock icon turns red, signaling potential security issues ahead. For sites without an encrypted connection, an unlocked red padlock icon appears, further emphasizing the lack of security. This functionality makes Certificate expiration a valuable tool for anyone concerned about online security.