The layout engine in Mozilla Firefox 3.x before 3.0.5 and 2.x before 2.0.0.19, Thunderbird 2.x before 2.0.0.19, and SeaMonkey 1.x before 1.1.14 allows remote attackers to cause a denial of service (crash) and possibly trigger memory corruption via vectors related to (1) a reachable assertion or (2) an integer overflow.

Per http://www.mozilla.org/security/announce/2008/mfsa2008-60.html

Mozilla developers identified and fixed several stability bugs in the browser engine used in Firefox and other Mozilla-based products. Some of these crashes showed evidence of memory corruption under certain circumstances and we presume that with enough effort at least some of these could be exploited to run arbitrary code.

Thunderbird shares the browser engine with Firefox and could be vulnerable if JavaScript were to be enabled in mail. This is not the default setting and we strongly discourage users from running JavaScript in mail. Without further investigation we cannot rule out the possibility that for some of these an attacker might be able to prepare memory for exploitation through some means other than JavaScript such as large images.