Well, I don’t know what the problem is exactly.
You could confirm that port 80 isn’t blocked by running
echo bonjour | nc -l 80
on your server, and then from another machine, running
nc www.mydomain.fr 80
and confirming that you see the word “bonjour”.
This isn’t using the HTTP protocol or any web technology, it’s using the TCP port directly to send some literal, uninterpreted data.