I was able to log into various stack exchange sites with no problem, but as of today 1/5/2016 I'm getting the following error message with firefox 43.0.2:
You have asked Firefox to connect securely to security.stackexchange.com, but we can't confirm that your connection is secure.
Normally, when you try to connect securely, sites will present trusted identification to prove that you are going to the right place. However, this site's identity can't be verified.
security.stackexchange.com uses an invalid security certificate. The certificate is not trusted because it was signed using a signature algorithm that was disabled because that algorithm is not secure. (Error code: sec_error_cert_signature_algorithm_disabled)
Firefox gives me the option to add an exception if I understand the risks. The problem is that I don't understand the risks. Is this problem unique to me? How can I fix this? (I've disabled all extensions in Firefox, but that didn't help. I'm posting the question as a guest since I couldn't log in.)
Update: as suggested in the comments, I checked the issuer of the certificate and it was "avast! Web/Mail Shield Root". After updating Avast to its latest version, the problem went away. Thanks everyone.