朋友们帮帮忙 javascript支持问题

以下代码IE遨游没问题，FIREFOX就不行，烦劳懂W3C的朋友帮忙看看，有没有可能改一下让它也支持FIREFOX呢，怎么改？我实在是个JS菜鸟

<script language="JavaScript" type="text/javascript">
        <!--
        function will()
        {  if
        (event.srcElement.tagName=='A'||event.srcElement.tagName=='IMG')
          {    window.focus();
               Rest.style.display ="";
               Ad.style.display = "none";
               document.cookie='world_online=yes';
          }
        }
        
        function web__cookie(Name) {
          var search = Name + '='
          var returnvalue = '';
          if (document.cookie.length > 0) {
            offset = document.cookie.indexOf(search)
            if (offset != -1) { // if cookie exists
              offset += search.length
              // set index of beginning of value
              end = document.cookie.indexOf(';', offset);
              // set index of end of cookie value
              if (end == -1)
                 end = document.cookie.length;
              returnvalue=unescape(document.cookie.substring(offset, end))
              }
           }
          return returnvalue;
        }
        //-->
        </script>

        <div id="Ad">
        <table width=100%><tr> <td align=center>
            <span id="A" onclick="will()"><a href="" target=_blank  onclick="will()">点击链接</a></span>
        </td></tr></table>
        </div>
        <div id="Rest" style="display:none">    隐藏内容</div>

        <script language="Javascript">
        if (web__cookie('world_online') !='') {
          Rest.style.display = "";
          Ad.style.display = "none";
        }
        else{
          Rest.style.display = "none";
          Ad.style.display = "";
        }
        </script>

谢谢各位大虾！