you could give each element a unique id like:
<?php $furniture = mysql_query("SELECT * FROM tbl_furniture WHERE furniture_active = '1'");
$i=0 // init counter
while($row = mysql_fetch_array($furniture))
{...
...
<p class="furn_itemprice" id="price<?php echo $i++;?>">£<?php ec... // append counter to element id
...
and
var q = document.getElementById("quantity"+i).value;
Also you should not use mysql*
for new code, it's been superceeded by mysqli*
personally I prefer PDO
for php database connections