My menu is breaking (see last link) in Firefox 3.5. Any clues as to why?

http://design4dot.com/safenethomesafety/index.html

ref: main.css

.horizmenu li a {
    float: left;
    color: white;
    padding: 4px 10px 0 10px;
    text-decoration: none;
    border-right: 1px solid white;
    font-size:15px;
    height:33px;
}

adjust the padding

Thank you.