CVE 2011-3670
Mozilla Firefox before 3.6.26 and 4.x through 6.0, Thunderbird before 3.1.18 and 5.0 through 6.0, and SeaMonkey before 2.4 do not properly enforce the IPv6 literal address syntax, which allows remote attackers to obtain sensitive information by making XMLHttpRequest calls through a proxy and reading the error messages.
See the 
CVE page on Mitre.org
for more details.
