Mozilla Updater in Mozilla Firefox before 24.0, Firefox ESR 17.x before 17.0.9, Thunderbird before 24.0, Thunderbird ESR 17.x before 17.0.9, and SeaMonkey before 2.21 does not ensure exclusive access to a MAR file, which allows local users to gain privileges by creating a Trojan horse file after MAR signature verification but before MAR use.