intl/uconv/util/nsUnicodeDecodeHelper.cpp in Mozilla Firefox before 3.6.7 and Thunderbird before 3.1.1 inserts a U+FFFD sequence into text in certain circumstances involving undefined positions, which might make it easier for remote attackers to conduct cross-site scripting (XSS) attacks via crafted 8-bit text.
Mozilla Firefox before 3.0.13, and 3.5.x before 3.5.2, allows remote attackers to spoof the address bar, and possibly conduct phishing attacks, via a crafted web page that calls window.open with an invalid character in the URL, makes document.write calls to the resulting object, and then calls the stop method during the loading of the error page.