Heap-based buffer overflow in Mozilla Firefox 2.x before 2.0.0.1, 1.5.x before 1.5.0.9, Thunderbird before 1.5.0.9, and SeaMonkey before 1.0.7 allows remote attackers to cause a denial of service (crash) and possibly execute arbitrary code by setting the CSS cursor to certain images that cause an incorrect size calculation when converting to a Windows bitmap.