Mozilla Characters mapped to U+FFFD in 8 bit encodings cause subsequent character to vanish

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 crafte...