no, it is not kde4 or i would have the problem too, i only have firefox and it is not doing what you describe
i don't have the gtk theme mentioned but i have set all my gtk apps to use qtcurve settings from kde4
what you describe could be a addon not responding, a memory leak they call it(very common lately with some addons

)
try firefox without any addon to see if you can replicate the situation, then enable some addons to discard suspects