Thanks. There was no logs in either access.log nor error.log as the browser was simply not connecting the Apache server.
I discovered that the problem was Firefox-only. As most Firefox settings are invisible through the Tools > Option graphical interface, the trick is to go in Firefox config.
Here the HowTo :
1) In Firefox
address bar, type :
about:config
2) A warning displays. Accept to enter in the config.
3) Make a
right click in the list of key-values and choose "
New > Character String"
4) Enter the character string
[b]toolkit.networkmanager.disable[/b]
5) When a value is asked, type
[b]true[/b]
That’s all.
What helped me was this topic on Mozilla support : how to connect to localhost in offline mode?
http://support.mozilla.com/tiki-view_forum_thread.php?locale=ro&comments_parentId=126819&forumId=1
Note: My config is Windows XP and Firefox 3. But I assume that it should work with other OS and Firefox versions.