The best solution is this:
@Derelict:
Install a certificate signed by a trusted root or tell your browser to trust the certificate.
Big problem ;) : it's won't be 'free'.
I bought a domain name, like 'my-domain.tld' (give or take a few $ a year). Then, visit startssl.com to obtain a free valid signed certificate for 'my-domain.tld' and "portal.my-domain.tld" (I did not take the included 'www.my-domaine.tld').
You get all the files needed to install into pfSense.
Switch portal authentication to https - no errors for all browsers. Works for me for years now.