Firefox 4.x delay on Opensuse 11.2

I am facing an issue with Firefox 4 (but it already existed with 3.6.x).

I have a local php application which I am using for my knowledgebase. When ever I start to use the application for a while I am facing massive delays in Firefox.

This means when I do an activity (scrolling, typing, opening a window or a tab, push the back button e.g.) I will have a delay of 1/2 second up to 3 - 4 seconds, before Firefox is reacting. FF is then working for a couple of seconds normally before I am facing the delay again.
I can close FF and restart again without any change of behaving. The only thing I can do to get rid of the delay is to reboot the system.

I am not sure if it is related to the graphical interface but I am using ATI Radeon 4670 and I just checked the Xorg.log if the system is using the interface correctly (it seems so, I could not find any indication of error or so). No desktop effects are enabled.
I am using Gnome 2.28.2 as the desktop. I am running FF 4 also on my notebook and there I do not have any issue when I am suing the php application.

Anyone any idea what this could be?