Firefox is probably running under a normal system account and does not have permission to save where you want it to. Either run firefox as a root user or grant yourself permission to save in /var/www/html/
If I'm just messing around in Apache, I explicitly grant myself full privileges so I don't run into these nuisances. ie
chmod -R 777 /var/www/
Obviously you need to be careful with this if security is a concern...