firefox/thunderbird is vulnerable to man-in-the-middle attacks. The address bar can be spoofed by operating a proxy server that provides a 407 HTTP status code accompanied by a malicious web script.
lists.opensuse.org/opensuse-security-announce/2013-02/msg00017.html
lists.opensuse.org/opensuse-updates/2013-02/msg00062.html
rhn.redhat.com/errata/RHSA-2013-0271.html
rhn.redhat.com/errata/RHSA-2013-0272.html
www.debian.org/security/2013/dsa-2699
www.mozilla.org/security/announce/2013/mfsa2013-27.html
www.mozilla.org/security/known-vulnerabilities/firefoxESR.html
www.ubuntu.com/usn/USN-1729-1
www.ubuntu.com/usn/USN-1729-2
www.ubuntu.com/usn/USN-1748-1
access.redhat.com/knowledge/solutions/294303
access.redhat.com/security/updates/classification/#critical
bugzilla.mozilla.org/show_bug.cgi?id=796475
oval.cisecurity.org/repository/search/definition/oval%3Aorg.mitre.oval%3Adef%3A16666
rhn.redhat.com/errata/RHSA-2013-0271.html