nsBaseAppShell::ScheduleSyncSection features the following: (@ http://dxr.mozilla.org/mozilla-central/widget/xpwidgets/nsBaseAppShell.cpp#l398) > // Ensure we've got a pending event, else the callbacks will never run. > if (!NS_HasPendingEvents(thread) && !DispatchDummyEvent(thread)) { > RunSyncSections(true, 0); > } Which means we could potentially run all sync events immediately within a call, which is generally not what the callee is intending. However, it does not seem like DispatchDummyEvent should be fallible in this case, so this should be replaced with an assertion or abort. (or the check dropped entirely)