I've been wrestling with this for awhile now. The problem is with firefox and the way that it caches iframe content. It's not random either. There is nothing to do to prevent this short of not using iframes.
You can reload the iframes onload using something like:
var reloadIframes = function () {
var a = window.frames, b = a.length
while (b--) {
a[b].src = a[b].src;
}
}
In the case of ads it will cause double impressions which will violate you contract.
An easy way to replicate the issue is create 3 html files.
<!--frame1.html-->
<html>
<body>
<h3>frame one</h3>
</body>
</html>
<!--frame2.html-->
<html>
<body>
<h3>frame two</h3>
</body>
</html>
<!--index.html-->
<html>
<body>
<iframe src="frame1.html"></iframe>
<iframe src="frame2.html"></iframe>
</body>
</html>
Open in firefox. Then switch frame one and frame two.
<!--index.html-->
<html>
<body>
<iframe src="frame2.html"></iframe>
<iframe src="frame1.html"></iframe>
</body>
</html>
Refresh index.html. The iframes will not be swapped until you clear your cache.
There is a bug in at mozilla but no one is currently working on it.