Mozilla Firefox and Thunderbird before 1.5.0.4 strip the Unicode Byte-order-Mark (BOM) from a UTF-8 page before the page is passed to the parser, which allows remote attackers to conduct cross-site scripting (XSS) attacks via a BOM sequence in the middle of a dangerous tag such as SCRIPT.
The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users.