CVE 2009-0772
The layout engine in Mozilla Firefox 2 and 3 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 vectors related to nsCSSStyleSheet
See the
CVE page on Mitre.org
for more details.