Comment 26 for bug 564181

(In reply to comment #22)
> (In reply to comment #21)
>
> > I've managed to find a way to trigger this - just use flash -
> > something which doesn't usually happen as I run flashblock. Unblocking a flash
> > totally trashes perf even after seamonkey is closed
>
> More testing shows that not just any flash will trigger it, but this one does -
>
> http://www.speedtest.bbmax.co.uk/
>

Might be related to bug #15293. In that case the performance hit was caused by flash reading back the video in order to draw stuff (e.g. the controls) over it.
I did see a bit of activity in pixman, though it was nowhere near what you're seeing.