You were right ! Thank you !
I installed firebug (by the way excellent tool! Thank’s for the recommendation !) and I figured out that the bug was inside the aside tag and specifically the width parameter.
But how can the width parameter affect the button of a menu? And how can I change it ? It is vital to specify the width of the aside.
aside {
float: left;
margin: 150px 0 0;
width: 181px;
}