As reported on secunia
http://secunia.com/advisories/13687/

Maurycy Prodeus has reported a vulnerability in Mozilla, which can be exploited
by malicious people to cause a DoS (Denial of Service) and potentially
compromise a user's system.

The vulnerability is caused due to a boundary error in the
"MSG_UnEscapeSearchUrl()" function in "nsNNTPProtocol.cpp" when processing NNTP
URIs. This can be exploited via e.g. a malicious web site to cause a heap-based
buffer overflow when referencing a specially crafted, overly long "news://" URI.

Successful exploitation crashes the application, but may potentially allow
execution of arbitrary code.

The vulnerability has been reported in version 1.7.3 and prior.