For the last few weeks I keep getting this error message: 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. Error code: SEC_ERROR_REUSED_ISSUER_AND_SERIAL It will suddenly happen for multiple sites that I have been connected to in the past, or new sites I try to open including support.mozilla.org. A solution is to clear my history and restart the browser. It only happens on my windows 10 laptop. I think happens when I have logged off or the device has slept and then firefox has restarted and opened previously open tabs. Other devices on the same network don't get effected.