alright for some reason when looking in developer tools in firefox the code in /js/jquery.init.js was missing for some reason but I managed to take a look at it by just opening it in the browser
anyways, the countdown code works by using setInterval, which is pretty inaccurate for timing and since firefox is trying to be clever by slowing down processing cycles in inactive tabs, the setInterval delay is actually being increased by the browser depending on processes and things
here's the actual code, run after document.ready
the same issue is also described in this stackoverflow question
== note: this could be a way to do this, might not be the best way and I haven't actually tested it, or know it will work ==
one way to fix this would be to base the timer off timestamps, get a UTC timestamp from the back-end php script and keep the setInterval. then every time HandleCountdownTimers is called, instead of decrementing the time by one, compare the php timestamp to Math.floor(Date.now() / 1000);
which gets the UTC timestamp in seconds. Find the second difference and return <length of compo in seconds> - <difference between timestamps> then parse seconds, minutes, hours, etc.