A possible reason for this is spyware on your computer, so before we making too many exceptions, let's investigate. If you could give me a few minutes to check this out:

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?

Also check the date and time in the clock on your computer: (double) click the clock icon on the Windows Taskbar.

• https://support.mozilla.org/kb/Secure+Connection+Failed

You can check the connection settings.

• Tools > Options > Advanced > Network Connection > Settings
• https://support.mozilla.org/kb/Options+window+-+Advanced+panel

If you do not need to use a proxy to connect to internet then try to select "No Proxy" if "Use the system proxy settings" or one of the others do not work properly.

See "Firefox connection settings":

• https://support.mozilla.org/kb/Firefox+cannot+load+websites+but+other+programs+can