Mozilla Firefox 3.6.3 on Windows XP SP3 allows remote attackers to cause a denial of service (memory consumption, out-of-bounds read, and application crash) via JavaScript code that appends long strings to the content of a P element, and performs certain other string concatenation and substring operations, related to the DoubleWideCharMappedString class in USP10.dll and the gfxWindowsFontGroup::GetUnderlineOffset function in xul.dll, a different vulnerability than CVE-2009-1571.