CVE 2011-3001
Mozilla Firefox 4.x through 6, Thunderbird before 7.0, and SeaMonkey before 2.4 do not prevent manual add-on installation in response to the holding of the Enter key, which allows user-assisted remote attackers to bypass intended access restrictions via a crafted web site that triggers an unspecified internal error.
See the 
CVE page on Mitre.org
for more details.
