รันใน chrome ไม่ขึ้นคับ แต่รันใน firefox ขึ้น รบกวนดูให้หน่อยคัย

<script src="http://code.jquery.com/jquery-latest.js"></script>
<script type="text/javascript">
	$(document).ready(function() {
		$("#txt5").change(function(){
			var brand = $("option:selected",this).val();
			$("#txt9 option").show();
			$("#txt9 option").each(function(){
			if($(this).attr("ref") != brand)
				$(this).hide();
			});
		});
	});
</script>

 ยี่ห้อรถ
      <select name="txt5" id="txt5">
    	<option value="">กรุณาเลือก</option>
        <option value="Aston">Aston</option>
        <option value="Bentley">Bentley</option>
        <option value="Daimler">Daimler</option>
        <option value="Ferrari">Ferrari</option>
        <option value="Jaguar">Jaguar</option>
        <option value="MG">MG</option>
        <option value="Nissan">Nissan</option>
        <option value="Porsche">Porsche</option>
        <option value="Rolls-Royce">Rolls-Royce</option>
     	<option value="Alfa">Alfa</option>
        <option value="Audi">Audi</option>
        <option value="BMW">BMW</option>
        <option value="Chrysler">Chrysler</option>
        <option value="Chevrolet">Chevrolet</option>
        <option value="Citroen">Citroen</option>
        <option value="Daihatsu">Daihatsu</option>
        <option value="Flat">Flat</option>
        <option value="Ford">Ford</option>
        <option value="Honda">Honda</option>
        <option value="Hyundai">Hyundai</option>
        <option value="Holden">Holden</option>
        <option value="Isuzu">Isuzu</option>
        <option value="Jeep Cherokee">Jeep Cherokee</option>
        <option value="Kai">Kai</option>
        <option value="Land Rover">Land Rover</option>
        <option value="Lexus">Lexus</option>
        <option value="Magso">Magso</option>
        <option value="Mazda">Mazda</option>
        <option value="Mercedes-Benz">Mercedes-Benz</option>
        <option value="Mitsubishi">Mitsubishi</option>
        <option value="Opel">Opel</option>
        <option value="Peugeot">Peugeot</option>
        <option value="Proton">Proton</option>
        <option value="Rover">Rover</option>
        <option value="Saab">Saab</option>
        <option value="Seat">Seat</option>
        <option value="Subaru">Subaru</option>
        <option value="Suzuki">Suzuki</option>
        <option value="Toyota">Toyota</option>
        <option value="Volkswagen">Volkswagen</option>
        <option value="Volvo">Volvo</option>
      </select>
รุ่นรถ &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
      <select name="txt9" size="1" id="txt9">
        <option value="">กรุณาเลือก</option>
        <option value="Martin" ref="Aston">Martin</option>
        <option value="Legend" ref="Honda">Legend</option>
        <option value="Nsx" ref="Honda">Nsx</option>
        <option value="Odyssey" ref="Honda">Odyssey</option>
        <option value="Accord" ref="Honda">Accord</option>
        <option value="Prelude" ref="Honda">Prelude</option>
        <option value="Civic" ref="Honda">Civic</option>
        <option value="CRV" ref="Honda">CRV</option>
        <option value="President" ref="Nissan">President</option>
        <option value="D-Max" ref="Isuzu">D-Max</option>
        <option value="Trooper" ref="Isuzu">Trooper</option>
        <option value="V-cross" ref="Isuzu">V-Cross</option>
        <option value="Cameo" ref="Isuzu">Cameo</option>
        <option value="Rodeo" ref="Isuzu">Rodeo</option>
        <option value="Vega" ref="Isuzu">Vega</option>
        <option value="Vertex" ref="Isuzu">Vertex</option>
        <option value="Mu-7" ref="Isuzu">Mu-7</option>
        <option value="Cab4" ref="Isuzu">Cab4</option>
	<option value="Vigo-Champ" ref="Toyota">Vigo-Champ</option>
        <!-- select ไม่ขึ้นเลยคับหลังจากบรรทัดนี้  -->
        <option value="Supra" ref="Toyota">Supra</option>
        <option value="Romeo" ref="Alfa">Romeo</option>
        <option value="Mx5" ref="Mazda">Mx5</option>    
        <option value="Pajero" ref="Mitsubishi">Pajero</option>  
        <option value="Calibra" ref="Opel">Calibra</option>
        <option value="Omaga" ref="Opel">Omaga</option>
        <option value="Land Cruiser" ref="Opel">Land Cruiser</option>
        <option value="Rav 4" ref="Opel">Rav 4</option>
        <option value="Zafira" ref="Chevrolet">Zafira</option> 
        <option value="Crus" ref="Chevrolet">Crus</option>
        <option value="Gran" ref="Daihatsu">Gran</option>
        <option value="Terios" ref="Daihatsu">Terios</option>
        <option value="Focus" ref="Ford">Focus</option>
        <option value="Festra" ref="Ford">Festra</option>
      </select>