Use-after-free vulnerability in the nsTreeSelection implementation in Mozilla Firefox before 3.0.19 and 3.5.x before 3.5.9, Thunderbird before 3.0.4, and SeaMonkey before 2.0.4 allows remote attackers to execute arbitrary code or cause a denial of service (application crash) via unspecified vectors that trigger a call to the handler for the select event for XUL tree items.