Heap-based buffer overflow in the nsHTMLReflowState::CalculateHypotheticalBox function in Mozilla Firefox 4.x through 12.0, Firefox ESR 10.x before 10.0.5, Thunderbird 5.0 through 12.0, Thunderbird ESR 10.x before 10.0.5, and SeaMonkey before 2.10 allows remote attackers to execute arbitrary code by resizing a window displaying absolutely positioned and relatively positioned elements in nested columns.