Mozilla developers identified and fixed several memory safety bugs in the browser engine used in Firefox and other Mozilla-based products. Some of these bugs 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.
This has been fixed in:
Firefox ESR 17.0.9
Thunderbird ESR 17.0.9
More information: http://www.mozilla.org/security/announce/2013/mfsa2013-76.html