Firefox may say it won't allow an exception either because the site has asked browsers to pre-load them as a Strict Transport Security site, or because on a previous visit to the site, it told Firefox never to visit on an insecure connection.

If the problem is an expired certificate, usually this affects all browsers and the site will be told quickly they need to fix it.

If the problem is a revoked certificate, users of other browsers may not be aware of this because Firefox is the last browser to check that in real time.

Could you copy/paste the error detail into a reply to see more exactly what is going on?

Are you using a proxy or VPN?

You can check the connection settings.

• Settings -> General -> Network: Connection -> Settings

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/en-US/kb/websites-dont-load-troubleshoot-and-fix-errors
• https://support.mozilla.org/en-US/kb/firefox-cant-load-websites-other-browsers-can
• https://support.mozilla.org/en-US/kb/firefox-and-other-browsers-cant-load-websites