Click advance then add exception on your browser also if your running a third party antivirus u mite need to configure it.
It has nothing to do with any firewall. This is a problem with your Firefox user profile. Simply put (simply to say), it's damaged. Old (expired) certificates have not been removed, cookies are uncleaned (likely to be damaged), etc. So, clean your Firefox userprofile or make a new one (please note that only can stay there the bookmarks (places.sqlite) file if it isn't damaged)