Is this with all sites, many, some? Is the computer you posted with the one with the problem?

Check out these links; https://www.bing.com/search?q=sec_error_reused_issuer_and_serial

sec_error_reused_issuer_and_serial. The cause is that the self generated certificate of the site you are browsing have the same serial number of a previously stored certificate (another Web site you already visited and manually added an exception for).

Try to rename the cert8.db file (cert8.db.old) and delete cert_override.txt in the Firefox profile folder to remove intermediate certificates and exceptions that Firefox has stored.

If that has helped to solve the problem then you can remove the renamed cert8.db.old file.

Firefox will store intermediate certificates that a server sends in the Certificate Manager for future use.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox