If you are trying to use or download the Firefox browser, you may In rare cases see something like the following message appear.
This Connection is Untrusted
You have asked Firefox to connect securely to www.mozilla.org, 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.
If you see a message like this, do not be alarmed. Just click the "Add Exception ... " button and follow the instructions from Mozilla Firefox support site: https://support.mozilla.org/en-US/questions/991547 (see image below).
If you already have Firefox it may be because you need to upgrade to a newer version.