^ I've notice this happens from time to time. I've not said anything because it wasn't as often. But now that you've done some tweaking its happening a little bit more.
Using Firefox 21.0a2 on Windows 8 if that is any help to you.
I've not been able to reproduce this. I do note that you're using the alpha channel of FF though. I'd also recommend confirming that you can reproduce it without any FF add-ons (particularly ones like ad blockers).