Site won't load on Firefox anymore - cert problem

I tried clearing cache & cookies; didn’t help.

Secure Connection Failed
An error occurred during a connection to You are attempting to import a cert with the same issuer/serial as an existing cert, but that is not the same cert.
• The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.
• Please contact the website owners to inform them of this problem.
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