Mozilla changed how you could add an “exception” certificate to the Firefox Certificate Manager when you see this dialog window

To add an exception certificate, follow these steps:

1. Open the Firefox Preferences control
2. Click the Advanced icon
3. Click the Encryption tab
4. Click the View Certificates button

   

5. Click the Servers tab
6. Click the Add Exceptions… button
7. In the Location: field, type the S3 bucketname + server’s URL and click the Get Certificate button

   

8. Wait as the Certificate Manager checks the Certificate Status
9. In a moment Wrong Site and Unknown Identity errors appear in the Certificate Status.
10. Place a checkmark next to Permanently store this exception
11. Click the Confirm Security Exception button.

    

12. Check that your “exception” certificate has been added:

    

13. Click the OK button and close the preferences window.
14. You should be able to access or add content to the S3 bucket now.