For the past several days I am getting a pop-up request for a security exception for one of email accounts in Mac Thunderbird. It shows up completely random and other than being a nuisance it doesn't seem to affect mail. I have a choice of clicking "confirm security exception" or "cancel". Regardless of what I click the pop-up window goes away and then comes back. Initially it was every 5-10 min., now it is every 30 min or so. Have rebooted the computer twice since this started. Any suggestions would be appreciated.