Chrome / Firefox之间的Javascript怪异

Question

我有一块JS正在通过php插入一些（秒）并为用户生成倒计时。它在Chrome中效果很好，但在Firefox中似乎总是为seconds_left设置零值。我错过了什么？

<script>
// variables for time units
var minutes, seconds;
var seconds_left = {COUNTDOWN};

// get tag element
var countdown = document.getElementById("countdown");

// update the tag with id "countdown" every 1 second
setInterval(function () {

minutes = parseInt(seconds_left / 60);
seconds = parseInt(seconds_left % 60);

// format countdown string + set tag value
if(seconds < 10) {
countdown.innerHTML = minutes + ":" + ("0" + seconds).slice(-2);
} else {
countdown.innerHTML = minutes + ":" + seconds;
}



if(seconds_left > 0){seconds_left--;}else{countdown.innerHTML = "NOW!";}  

}, 999);
</script>