function checkBrowserName(name){  
    var agent = navigator.userAgent.toLowerCase();  
    if (agent.indexOf(name.toLowerCase())>-1) {  
      return true;  
    }  
    return false;  
  }  
var vHeight = document.getElementById("mainnav").offsetHeight;

 if(checkBrowserName('firefox')){  
   vHeight = vHeight*2;  
 }  
  
document.getElementById("leftsite").style.height = Hoehe-100-vHeight-10+"px";
document.getElementById("nav2").style.height = Hoehe-100-vHeight-135+"px";
document.getElementById("main").style.height = Hoehe-100-vHeight-10+"px";
document.getElementById("main").style.width = Weite-200-175+"px";
document.getElementById("rightsite").style.height = Hoehe-100-vHeight-10+"px";

document.getElementById("leftsite").style.top = 100+vHeight+"px";
document.getElementById("main").style.top = 100+vHeight+"px";
document.getElementById("rightsite").style.top = 100+vHeight+"px";