I am using KDE3! With basic compositing in kwin like transparency and shadows, nothing else

But I can try to disable that. I don't think Mesa 8 has any effect (but I am using it), as we concluded FF is not using OpenGL. I am on the latest radeon driver. But my X server is quite old (like 1.9), maybe that is the problem. I still think your CPU makes it faster for you (I suppose you also see FF using 100% of your CPU core) as I only have a 3Ghz Phenom II 960.
What version of Firefox have you tested there?