Expiry Canary

Expiry Canary is a Firefox 3 or later, or SeaMonkey 2, add-on to warn you about certificates which are about to expire. You can alter the warning period and the sites on which it activates, allowing site admins to have a backstop warning about certificate expiry and prevent embarrassing accidents (Google, Yahoo, LinkedIn).

You can find it on addons.mozilla.org for Firefox 3+ or Seamonkey 2.