The user interface event dispatcher in Mozilla Firefox 3.0.3 on Windows XP SP2 allows remote attackers to cause a denial of service (NULL pointer dereference and application crash) via a series of keypress, click, onkeydown, onkeyup, onmousedown, and onmouseup events.  NOTE: it was later reported that Firefox 3.0.2 on Mac OS X 10.5 is also affected.