Firefox double counter problem

I'm implementing a very simple 'self-configuring' counter script that essentially creates a counter for every page it's used on with the option to update the counter or not and to return or display the page's counter. It interfaces to a simple, one-table database with only two columns.

define("NOTSET",-1024);
$server = "server";
$user = "user";
$password = "password";
$db = "db";
$link = mysql_connect($server,$user,$password) or die("Cannot establish connection with counter server");
mysql_select_db($db) or die("Cannot select database");

//returns the CID (table col) hash according to $id
// (id should be the SCRIPT_FILENAME of the requesting page)
function counterHash($id) {
  $md5 = md5($id); // md5 is relatively surjective
  return substr($md5,0,10);
  }
function getCount($hash) {
  global $link;
  $safe = mysql_real_escape_string($hash);
  $query = "SELECT `count` FROM `counters` WHERE `CID`='".$safe."'";
  $result = mysql_query($query);
  if ( ! is_resource($result) ) { print("notset!"); return NOTSET; }
  $row = mysql_fetch_array($result,MYSQL_ASSOC);
  return $row['count'];
  }

// If there isn't a row for $hash in the db, then one is
// created and false is returned.
// Otherwise, returns true.
function createRowForHashIfNotExists($hash) {
  global $link;
  $safe = mysql_real_escape_string($hash);
  $q = "SELECT `count` FROM `counters` WHERE `CID`='$safe'";
  $result = mysql_query($q) or die("Cannot get status because ".mysql_error() );
  $n = mysql_num_rows($result);
  if ( $n === 0 ) { // no rows for our hash
    $q = "INSERT INTO `counters` (`CID`,`count`) VALUES('$safe','0')";
    mysql_query($q) or die("cannot create new counter because ".mysql_error() );
    return false;
    }
  }

//sets the `count` col of $hash's row to $count
function setCount($hash,$count) {
  global $link;
  createRowForHashIfNotExists($hash);
  $safehash = mysql_real_escape_string($hash);
  $safecount = (int)mysql_real_escape_string( (int)$count );
  if ( ! is_int($safecount) ) { die('setcount(hash,INTEGER)'); }
  $query = "UPDATE `counters` SET `count` ='".$safecount."' WHERE `CID` = '".$hash."' LIMIT 1";
  mysql_query($query) or die("Cannot update counter because " . mysql_error() );
  }

$id = $_SERVER['SCRIPT_FILENAME'];
$hash = counterHash($id);
createRowForHashIfNotExists($hash);
$visitorCounter = getCount($hash);

function visitorCounter( $update=true, $show=true ) {
  global $link, $visitorCounter, $hash;
  if ( $update ) {
    setCount($hash,++$visitorCounter);
    }
  if ( $show ) {
    echo '

'.$visitorCounter.' Visitors

'."\n";
    } else { // don't show so return
      return $visitorCounter;
      }
    return true;
  }

?>

Everything works just fine in Safari (i.e. the page is visited and the counter increments only once), but in Firefox, the counter increments twice for no apparent reason. I figured this out only after wasting like three hours tracing through the program and reconstructing everything from scratch.

Is anybody familiar with what might be causing this? Is there a glaring problem with my code? Or, perhaps, most importantly, do you happen to see a fix or workaround? I could just do a browser detection and then not increment if firefox, but that's really more like a bad hack than an effective workaround...