CVE 2009-2462
The browser engine in Mozilla Firefox before 3.0.12 and Thunderbird allows remote attackers to cause a denial of service (memory corruption and application crash) or possibly execute arbitrary code via vectors related to (1) the frame chain and synchronous events, (2) a SetMayHaveFrame assertion and nsCSSFrameConst
See the
CVE page on Mitre.org
for more details.