One possible reason for this is if your system clock has an incorrect date, time, or time zone. That's fairly easy to check/fix is that is the issue. However, another possibility is spyware on your computer. Here's how to investigate that: Firefox may have presented an error page where there's a section that says "I understand the risk" where you can add an exception for the site. ''If you have added Exceptions for sites,'' 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. To view the certificate on a site where you made an exception, click the padlock in the address bar > More Information > View Certificate To view exceptions without going back to the site, you can use: "3-bar" menu button (or Tools menu) > Options > Advanced > Certificates mini-tab > "View Certificates" button > Servers tab then click a certificate and use the View button. One you have the viewer open, check the "Issued by" section. 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 any 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?