To reproduce, run the Mochitests in docshell: perl runtests.pl \ --setenv=XPCOM_DEBUG_BREAK=stack-and-abort \ --autorun \ --test-path=docshell \\ --close-when-done The resulting stack trace incriminates safebrowsing code: xpc3250!XPCWrappedNativeScope::FindInJSObjectScope+0x00000000000000A7 (c:\moz\trunk\mozilla\js\src\xpconnect\src\xpcwrappednativescope.cpp, line 718) xpc3250!XPCConvert::NativeInterface2JSObject+0x00000000000000B8 (c:\moz\trunk\mozilla\js\src\xpconnect\src\xpcconvert.cpp, line 1093) xpc3250!XPCConvert::NativeData2JS+0x0000000000000791 (c:\moz\trunk\mozilla\js\src\xpconnect\src\xpcconvert.cpp, line 481) xpc3250!nsXPCWrappedJSClass::CallMethod+0x0000000000000B05 (c:\moz\trunk\mozilla\js\src\xpconnect\src\xpcwrappedjsclass.cpp, line 1340) xpc3250!nsXPCWrappedJS::CallMethod+0x0000000000000041 (c:\moz\trunk\mozilla\js\src\xpconnect\src\xpcwrappedjs.cpp, line 568) xpcom_core!PrepareAndDispatch+0x0000000000000323 (c:\moz\trunk\mozilla\xpcom\reflect\xptcall\src\md\win32\xptcstubs.cpp, line 114) xpcom_core!SharedStub+0x0000000000000016 (c:\moz\trunk\mozilla\xpcom\reflect\xptcall\src\md\win32\xptcstubs.cpp, line 142) brwsrcmp!nsDocNavStartProgressListener::Observe+0x0000000000000308 (c:\moz\trunk\mozilla\browser\components\safebrowsing\src\nsdocnavstartprogresslistener.cpp, line 372) xpcom_core!nsTimerImpl::Fire+0x00000000000002C0 (c:\moz\trunk\mozilla\xpcom\threads\nstimerimpl.cpp, line 415) xpcom_core!nsTimerEvent::Run+0x00000000000000A1 (c:\moz\trunk\mozilla\xpcom\threads\nstimerimpl.cpp, line 489) xpcom_core!nsThread::ProcessNextEvent+0x00000000000001FA (c:\moz\trunk\mozilla\xpcom\threads\nsthread.cpp, line 491) xpcom_core!NS_ProcessNextEvent_P+0x0000000000000053 (c:\moz\trunk\mozilla\fx-dbg\xpcom\build\nsthreadutils.cpp, line 227) gkwidget!nsBaseAppShell::Run+0x000000000000005D (c:\moz\trunk\mozilla\widget\src\xpwidgets\nsbaseappshell.cpp, line 154) tkitcmps!nsAppStartup::Run+0x000000000000006B (c:\moz\trunk\mozilla\toolkit\components\startup\src\nsappstartup.cpp, line 170) xul!XRE_main+0x000000000000316B (c:\moz\trunk\mozilla\toolkit\xre\nsapprunner.cpp, line 3142) firefox!main+0x00000000000002B2 (c:\moz\trunk\mozilla\browser\app\nsbrowserapp.cpp, line 153)a firefox!__tmainCRTStartup+0x00000000000001A6 (f:\rtm\vctools\crt_bld\self_x86\crt\src\crtexe.c, line 586) firefox!mainCRTStartup+0x000000000000000D (f:\rtm\vctools\crt_bld\self_x86\crt\src\crtexe.c, line 403) kernel32!RegisterWaitForInputIdle+0x0000000000000049 (firefox!mainCRTStartup+0x000000000000000D (f:\rtm\vctools\crt_bld\self_x86\crt\src\crtexe.c, line 403

It appears that all 30-odd instances of this particular assertion in Mochitests have this stack trace; see the slightly-unbuffered output in bug 404077.

I shall boldly predict that this is no longer an issue, and that at a minimum this isn't likely we're suddenly going to fix based upon the existence of a bug. Feel free to reopen if that's not the case!