I had this with my old laptop and thought that is my "overloaded" FireFox with all those extensions. However, that continued with my new notebook as well. Now, I noticed this is not only FireFox but also the same thing happens when I use Internet Explorer (which I use only then when I need to open the same ApEx application twice).
See the picture below. It looks like while loading the forum the page is not rendered completely. This behaviour is only there when using Oracle Forums and they happen in both cases - while loading the main page or a single thread. Therefor my question to all readers of this blog: have you ever noticed something like that? I have this from time to time regardless of the connection I use. Currently, I use DSL 16000.