This add-on checks the certificates received by Firefox from SSL enabled websites and compares their thumbprints to the thumbprints SignatureCheck.org receives from the same sites. It will then alert the user if a different thumbprint is detected.
This can detect man-in-the-middle attacks that use valid signing certificates because the connection to SignatureCheck.org is protected by a key independent of the PKI trusted by the browser.
It can also check self-signed certificates to provide validation for websites that use them.
For more information, visit SignatureCheck.org