There are a few possible reasons for this. Some security suites include a filtering feature. In order to filter secure connections (HTTPS URLs), the security software presents a fake certificate to Firefox so it can intercept and stand in the middle of the secure connection. To have Firefox trust these certificates, you may need to do something such as import a root certificate, or click something in your security software's settings. But I'm not aware of Norton causing this problem, so I'm suspicious that it's something less benign. ''If you have added Exceptions,'' please check the "Issued by" section on one or two of the exceptions you've made to learn more about the source of this issue. orange Firefox button (or Tools menu) > Options > Advanced > Certificates mini-tab > "View Certificates" button > Servers tab Click a certificate and use the View button. The kind of issuer you might find is: * Name associated with your security software, such as ESET, BitDefender, etc. * Sendori (indicates unwanted software from Sendori) * FiddlerRoot (indicates unwanted software named similarly to BrowserSafeguard, BrowserSafe, SafeGuard) * Something else ''If you have not added Exceptions,'' you can click the Add Exception button in the error page, then in the dialog click View Certificate or Get Certificate to see the Issued by section. You do not need to finish adding an exception. What do you see?