Javascript compatibility issue, not running on Firefox

Question

I am trying to show tooltip on mouseover in asp.net (vb.net). However it is working on chrome, IE but not working on Firefox.

Below is my code:

 function getData(arg){ 
    var process=arg.id;
    var value;

    document.mouseover=function(event){
        event=event||window.event;
        event=event.clientX;

        var mouseX;
        var mouseX;
        var str;
        var showTip=true;

        if(process.indexOf('imgProcessOne')>1){
           Value = arg.parentElement.nextSibling.textContent;
           if(value=="N"){ showTip=false;}
        }
        if(process.indexOf('imgProcessTwo')>1){
           Value = arg.parentElement.nextSibling.nextSibling.textContent;
           if(value=="N"){ showTip=false;}
        }
        if(process.indexOf('imgProcessThree')>1){
           Value =  arg.parentElement.nextSibling.nextSibling.nextSibling.textContent;
           if(value=="N"){ showTip=false;}
        }
       if(process.indexOf('imgProcessFour')>1){
           Value = arg.parentElement.nextSibling.nextSibling.nextSibling.nextSibling.textContent;
           if(value=="N"){ showTip=false;}
        }
       if(showTip==true)
          str="<table class='className' style='background-       image:url(../../../images/tooltip1.png)'><tr><td>"+value+"</td></tr></table>";
          $('.toolTip .content').html(str).css('background','transparent'); 
          $('toolTip').css('left', event.clientX - 40).css('top', event.clientY - 70);
          $('.toolTip').css('display','block');
    }
    else{ return false; }