This problem appeared a couple of nightly build back and has been reported multiple times via the automatic crash feedback message.
Its not an immediate reaction but always occurs while trying to scroll the screen up after a few minutes of browsing.
Clearing cache and resetting Webkit has no effect.
I'm seeing the same since August 3rd.
Crashed process in WebCore::FrameView::scrollContentsFastPath
Created attachment 156851 [details]
Created attachment 156852 [details]
Looks like a duplicate of 93199. It has been fixed by r124811.
*** This bug has been marked as a duplicate of bug 93199 ***