Just to make sure:
Check the date and time in the clock on your computer: (double) click the clock icon on the Windows Taskbar. See Secure connection failed and Firefox did not connect

If you can't add an exception, but get a pop-up with the error message then you can check the pref browser.xul.error_pages.enabled on the about:config page and make sure that the value is set to the true (default).

Also make sure that you are not in (permanent) Private Browsing - Use Firefox without saving history mode.

Apparently there is no way of visiting a secured website if its certificate has expired. I have one which expired one week ago; my system date/time is correct and I cannot get past the security warning message, even if "I understand the risks".

I cannot add an exception because the certificate is valid, says the message, but the certificate is not valid anymore! I had the problem with Firefox 6 and I still have it with Firefox 7.0.1.

Su-u-uper. Nici nu mi-am inchipuit ca o buturuga atat de mica e in stare sa rastoarne un car atat de mare.

English, please.