Firefox and Chrome are both very strict these days regarding security.
If they are warning about an insecure site, it usually means one of two things - either the site has incorrectly configured their SSL certificate (the thing that enables them to use HTTPS and encrypt web traffic), or your computer clock is wrong and the browser thinks the certificate has already expired.
If it is a site you know and trust, usually there is an option to add a temporary exception below the warning (it might be a small link saying "Add Exception" or "Advanced Options", etc.). Do not do this unless you trust the site.
