I'm running Firefox 23.0 (updated it just today from 22.0) so I don't think that is this the problem and i don't think that someone is running a MITM attack right now 
I don't want to go Off-Topic anymore but anyway I've appreciated your help, thanks.
I don't want to go Off-Topic anymore but anyway I've appreciated your help, thanks.
It's probably due to him not linking his signed public rsa key with his certificate providers key in the web server. Firefox tends to need this, while chrome already has the certificate providers key built-in.