I do not see any problem with the website's certificate (screenshot attached).

Does the error page have any technical details about why Firefox won't load the site? I don't know if Firefox Focus security error pages are similar to Firefox error pages, but you could take a look at this article for comparison: What do the security warning codes mean?

This website I'm trying to open is not a malicious site. It is my Blue Cross Blue Shield Healthcare Plan. Firefox is recognizing it as a malicious unverifiable site. Here is what it is telling me:

Secure Connection Failed

The connection to the server was reset while the page was loading.

   The page you are trying to view cannot be shown because the authenticity of the received data could not be verified.
   Please contact the website owners to inform them of this problem.

Learn more…

Report errors like this to help Mozilla identify and block malicious sites

Sometimes that just means that Firefox couldn't connect. Does this article help? Secure connection and security warning error pages in Firefox