Thanks for the feedback. I have just received an email from the person I mentioned in my previous post, who was experiencing a similar issue, and it turns out that the issue disappeared after a re-installation of Windows. I am not suggesting you do that, however it is possible that the issue is due to your system configuration. Since it is only happening when you use Firefox, perhaps you could disable/remove all of the Firefox addons/plugins/extensions to see if that helps, or perhaps even uninstall and reinstall Firefox.
