Error:
You have received an invalid certificate. Please contact the server administrator or email
correspondent and give them the following information:
You certificate contains the same serial number as another certificate issued by the certificate
authority. Please get a new certificate containing a unique serial number.
Solution:
Open Firefox
From the top menu, click “Tools > Options”.
Click the “Advanced” icon.
Click the “Security” tab.
Click “View Certificates” button and then the “Web Sites” tab.
Delete the certificate.
Disable it in order to ping wifi hosts.