The browser engine in Mozilla Firefox before 3.0.12 and Thunderbird allows remote attackers to cause a denial of service (memory corruption and application crash) or possibly execute arbitrary code via vectors related to (1) the frame chain and synchronous events (2) a SetMayHaveFrame assertion and nsCSSFrameConstructor::CreateFloatingLetterFrame (3) nsCSSFrameConstructor::ConstructFrame (4) the child list and initial reflow (5) GetLastSpecialSibling (6) nsFrameManager::GetPrimaryFrameFor and MathML (7) nsFrame::GetBoxAscent (8) nsCSSFrameConstructor::AdjustParentFrame (9) nsDOMOfflineResourceList and (10) nsContentUtils::ComparePosition.