Guys.... been quite a while (sickness, other work to do, ...).
Here is the solution:
section, nav, header, footer {
display: block;
}
The problem was is related to mootools in any way, it's just a different
interpretation of the new elements in different browsers. I wanted to use
them as block elements, so I had to explicitly state so.
Thanks anyway for your help and the nifty tip about jsfiddle :-)
