You may see the following error when attempting to access a router's configuration page:

The error page will include the message, Your certificate contains the same serial number as another certificate issued by the certificate authority. Please get a new certificate containing a unique serial number. Error code: SEC_ERROR_REUSED_ISSUER_AND_SERIAL. This article describes the cause of this problem and how to work around it.

Cause

Older routers will sometimes improperly regenerate their security certificates instead of keeping the same one for the life of the device. If you have added a permanent security exception for your router, and if this regeneration is done improperly, Firefox will detect the change as a possible attack.

Solution

A long term solution for older routers is to contact your hardware vendor and see if updates are available for your device which fix this problem.

Workaround

If you are confident that no attack has occurred, you can work around the problem by removing previously saved exceptions generated by the same server or device. Most likely the exception with the duplicate serial number will be on the Servers tab. Look for ones you've added; ignore the ones that list * as the server, which are global distrust entries added by Firefox.

To delete your old exception:

1. In the Menu bar at the top of the screen, click Firefox and then select Preferences or Settings, depending on your macOS version.Click the menu button and select Settings.
2. Select the Privacy & Security panel.
3. Scroll down to the Certificates section.
4. Click View Certificates to open the Certificate Manager window.
5. In the Certificate Manager window click on the Servers tab.
6. Find the item that corresponds to the site that generates the error. Note the Certificate Authority (CA) for that server - the CA name appears above the site name.
7. Click on the server certificate that corresponds to the site that generates the error and press Delete....
8. Click OK when prompted to delete the exception.
9. Click on the Authorities tab.
10. Click on the item that corresponds to the CA that you noted earlier and then press Delete or Distrust....
11. Click OK when prompted to delete the exception.
12. Click OK close the Certificate Manager window.
13. Close the Settings page. Any changes you've made will automatically be saved.