The right start
As much as it pains me to say, knowing I'm a real firefox fan, the chrome error messages are WAY more self-explanatory than the crap firefox is displaying (as of 24 september 2014).
So for this part, stick to Chrome for debugging.
[Tip] non-www to www redirection with apache
Redirect every request without www subdomain to www subdomain.
The "domain name agnostic way".
RewriteEngine On
RewriteCond %{HTTP_HOST} !^www\.
RewriteRule ^(.*)$ http://www.%{HTTP_HOST}/$1 [R=301,L]