From OCS Support Wiki

Introduction

This guide will show you how to remove a stored SSL certificate in Firefox. It is necessary to do this sometimes if the certificate you have stored is corrupted or has changed in some way that prevents you from logging into an SSL site.

Removing the Certificate

To remove the certificate in Firefox:

• Click on the Tools menu and select Options
• Click on the Advanced tab and click View Certificates
• Click on the Websites tab
• Locate the certificate for the site in question, and click Delete
• Close all windows and restart Firefox

You should now be able to visit the SSL site.