The remote host is using a vulnerable version of Mozilla Firefox or Mozilla Thunderbird.
The remote host is using a version of Mozilla or Mozilla Thunderbird that is vulnerable to a heap overflow vulnerability. A heap overflow has been reported for the Mozilla POP3 mail handler routines. An attacker controlled POP3 server may be able to execute arbitrary code on the client machine.
Upgrade to Mozilla Firefox 1.7.1 / Thunderbird 0.7.1 or higher.