I was creating a site that had custom page transitions and had the issue that Firefox was display a flash of unwanted content (FOUC). It seems this is a long-standing bug. The fix that worked for me is the one from vrancken.gilbert who says running a dummy script right after the opening body tag will stop the FOUC.
So, this will fix things for you:
<body>
<script>0</script>
<!-- The rest of your content -->
</body>
Tim Bennett is a web designer and developer. He has a First Class Honours degree in Computing from
Leeds Metropolitan University and currently runs his own one-man web design company, Texelate.