Mozilla Firefox before 3.5.12 and 3.6.x before 3.6.9, Thunderbird before 3.0.7 and 3.1.x before 3.1.3, and SeaMonkey before 2.0.7 on Mac OS X allow remote attackers to cause a denial of service (memory corruption and application crash) or possibly execute arbitrary code via a crafted font in a data: URL.