Use-after-free vulnerability in the nsFrameList::FirstChild 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 or cause a denial of service (heap memory corruption and application crash) by changing the size of a container of absolutely positioned elements in a column.