CVE 2012-0452
Use-after-free vulnerability in Mozilla Firefox 10.x before 10.0.1, Thunderbird 10.x before 10.0.1, and SeaMonkey 2.7 allows remote attackers to cause a denial of service (application crash) or possibly execute arbitrary code via vectors that trigger failure of an nsXBLDocumentIn
See the
CVE page on Mitre.org
for more details.