Mozilla Firefox before 3.6.7 and Thunderbird before 3.1.1 do not properly implement read restrictions for CANVAS elements, which allows remote attackers to obtain sensitive cross-origin information via vectors involving reference retention and node deletion.
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A11887
https://bugzilla.mozilla.org/show_bug.cgi?id=571287
http://www.mozilla.org/security/announce/2010/mfsa2010-43.html