First make sure Firefox is the latest version. When the "Warning: Potential Security Risk Ahead page" in Firefox loads click advanced then select accept risks, it loads fine. The problem is after Mark`s passing no one knows the correct credentials at the moment. I would think that very soon it will all be sorted but it is OK and safe to carry on using it.
Apparently the SSL has a 90 day cert so it needs replacing every 90 days, hence the problem arising now. The warning is to let you know that something is wrong, well we know what is wrong and it`s nothing sinister.
HTH