The layout engine in Mozilla Firefox before 3.0.7, Thunderbird before 2.0.0.21, and SeaMonkey 1.1.15 allows remote attackers to cause a denial of service (crash) and possibly execute arbitrary code via certain vectors that trigger memory corruption and assertion failures.

More... (http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2009-0771)