CVE 2011-2991
The browser engine in Mozilla Firefox 4.x through 5, SeaMonkey 2.x before 2.3, Thunderbird before 6, and possibly other products does not properly implement JavaScript, which allows remote attackers to cause a denial of service (memory corruption and application crash) or possibly execute arbitrary code via unspecified vectors.
See the
CVE page on Mitre.org
for more details.