Javascript Jquery OS Browser detection

Javascript-Jquery

if (navigator.appVersion.indexOf("Win")!=-1){
     ff=navigator.userAgent.search("Firefox");
     if(ff>-1){
      $("#nav a").addClass('padding-right');
     }
 }
 else{
  $("#nav a").removeClass('padding-right');    
 }

CSS

body{
    font-family:arial;
}
#nav a{
    width:120px;
    height:40px;
    display:block;
    line-height:40px;
    background: #EA7A0C;
    color:#fff;
    text-align:center;
    font-size:13px;
    text-decoration: none;
}
.padding-right{
    padding-right:100px;
}