I’ve come accross this a few times, typically if a previous https site that you have been to forgets to renew their SSL certificate or if the certificate is self signed you can find that Firefox will not allow you to re-add a security exception.
You are in essence unable to view the site in Firefox at all. But, it is easily resolved.
Goto History, show all history.
Search for the offending site that you can’t access
Right click on the site and choose “Forget About This Site”
reload the site in the browser and you should now be able to store the security exception and browse it as normal.
