$(".box_yazi2").each(function() {
  var default_value = this.value;
  $(this).css('color', '#555'); // this could be in the style sheet instead
  $(this).focus(function() {
   if(this.value == default_value) {
    this.value = '';
    $(this).css('color', '#000');
   }
  });
  $(this).blur(function() {
   if(this.value == '') {
    $(this).css('color', '#555');
    this.value = default_value;
   }
  });
 });
 });

This function of default value of input doesnt work in FF, but perfectly works in IE and ofcourse the input itself looks like this:

<input type="text" class="box_yazi2" id="konu" name="konu" value="Bos" />