Hi Beth, could you copy/paste the error code and any other available detail into a reply? If the error page has an "Advanced" button, please click the button for more information.

For quick reference, this thread is:

https://support.mozilla.org/questions/1303941#question-reply

@Jscher2000

This was the response under the "Advanced" button. Someone could be trying to impersonate the site and you should not continue.

Websites prove their identity via certificates. Firefox does not trust mccsc.instructure.com because its certificate issuer is unknown, the certificate is self-signed, or the server is not sending the correct intermediate certificates.

Error code: SEC_ERROR_UNKNOWN_ISSUER

View Certificate

However, an option that hadn't appeared before (accept risk and continue) appeared this time. I didn't change any settings and don't know why it is appearing now, but I will take it. I am able to login and navigate the site. Thank you!

Making an exception is an option, but please don't use it on sites where you need to enter sensitive information, whether financial, health, or otherwise, without first investigating the source of the problem.