Cert Viewer Plus is a Thunderbird add-on that extends the certificate viewer dialog with additional options: an X.509 certificate can be displayed in PEM format (opens in a new window) or saved to a file (PEM/DER/PKCS#7). On the "General" tab, Cert Viewer Plus will also show any e-mail addresses included in the certificate, and make the fingerprint values copyable. On the "Details" tab, the trust configuration (SSL/Mail/Code) is shown for each certificate, and can be edited in newer releases of Firefox/Thunderbird/Seamonkey. Furthermore, an entry for direct access to the certificate manager dialog is added to the Tools menu, together with a keyboard shortcut.
This add-on is also available for Firefox.