I'm creating a site and I'm having a problem with the layers I have for the main page and the menu.

I have these in my CSS file :

PHP Code:
#site_top{
    margin: 0;
    padding: 10px 0px;
    border-bottom: 1px solid #cccccc;
    width: 100%;
}

#main_page { 
    margin-top: 0px;
    margin-left: 25%;
    margin-right: 0px;
}

#menu {
    float: left;
    width: 22%;
    background-color: #eeeeee;
} 
It all works fine but in Firefox (and opera), if the page is long and a scrollbar becomes visible, then the percentage I have is different and it makes the site_top layer and the menu layer smaller and the difference is visible when going from one option of the menu to another. In IE that the scrollbar is visible and either enabled or disabled this doesn't happen...

I'm sure someone has run into this before... Any solution other than the obvious (which I assume would work) to have a fixed (not %) width for the menu?

Thanks