Mozilla Firefox 3.0.5 and earlier 3.0.x versions, when designMode is enabled, allows remote attackers to cause a denial of service (NULL pointer dereference and application crash) via a certain (a) replaceChild or (b) removeChild call, followed by a (1) queryCommandValue, (2) queryCommandState, or (3) queryCommandIndeterm call.  NOTE: it was later reported that 3.0.6 and 3.0.7 are also affected.