Question

take a look at this site : IUT Wiki

A strange margin appears at the left side of the whole html and it only appears in Firefox.

Do you guys have any idea on how to fix this?

Answer 1

try:

body {
    overflow-x: hidden;
}

EDIT: If you want to solve the cause of the problem:

you have in the css:

ul#f-list {
    width: 1025px;
}

change it to 100%