The Mozilla Updater in Mozilla Firefox before 21.0 on Windows does not properly maintain Mozilla Maintenance Service registry entries in certain situations involving upgrades from older Firefox versions, which allows local users to gain privileges by leveraging write access to a "trusted path."
https://oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A17125
https://bugzilla.mozilla.org/show_bug.cgi?id=854088
http://www.mozilla.org/security/announce/2013/mfsa2013-45.html