Mozilla Firefox and Thunderbird before 1.5.0.4 allow remote attackers to cause a denial of service (crash) and possibly execute arbitrary code via (1) nested <option> tags in a select tag, (2) a DOMNodeRemoved mutation event, (3) "Content-implemented tree views," (4) BoxObjects, (5) the XBL implementation, (6) an iframe that attempts to remove itself, which leads to memory corruption.