During run of all offline mochitests I encoutered this deadlock on the main thread during simpleManifest test execution: nspr4.dll!PR_WaitCondVar(PRCondVar * cvar=0x06867d30, unsigned int timeout=0xffffffff) Line 551 + 0x17 bytes C > necko.dll!nsCacheRequest::WaitForValidation() Line 168 + 0xf bytes C++ necko.dll!nsCacheService::ProcessRequest(nsCacheRequest * request=0x04dcc9e0, int calledFromOpenCacheEntry=0x00000001, nsICacheEntryDescriptor * * result=0x0012eb6c) Line 1103 + 0x8 bytes C++ necko.dll!nsCacheService::OpenCacheEntry(nsCacheSession * session=0x063a6ac0, const nsACString_internal & key={...}, int accessRequested=0x00000001, int blockingMode=0x00000001, nsICacheListener * listener=0x00000000, nsICacheEntryDescriptor * * result=0x0012eb6c) Line 1168 + 0x15 bytes C++ necko.dll!nsCacheSession::OpenCacheEntry(const nsACString_internal & key={...}, int accessRequested=0x00000001, int blockingMode=0x00000001, nsICacheEntryDescriptor * * result=0x0012eb6c) Line 98 + 0x1b bytes C++ xpcom_core.dll!NS_InvokeByIndex_P(nsISupports * that=0x00000005, unsigned int methodIndex=0x00000004, unsigned int paramCount=0x0012eb3c, nsXPTCVariant * params=0x002fad5c) Line 102 C++ xpc3250.dll!XPCWrappedNative::CallMethod(XPCCallContext & ccx={...}, XPCWrappedNative::CallMode mode=0x00000005) Line 2393 + 0x21 bytes C++ xpc3250.dll!XPCWrappedNative::CallMethod(XPCCallContext & ccx={...}, XPCWrappedNative::CallMode mode=CALL_METHOD) Line 2393 + 0x21 bytes C++ xpc3250.dll!XPC_WN_CallMethod(JSContext * cx=0x067edea8, JSObject * obj=0x03e63d20, unsigned int argc=0x00000003, long * argv=0x061d0c34, long * vp=0x0012ee14) Line 1473 + 0xe bytes C++ js3250.dll!js_Invoke(JSContext * cx=0x067edea8, unsigned int argc=0x00000003, long * vp=0x061d0c2c, unsigned int flags=0x00000002) Line 1307 + 0x20 bytes C++ js3250.dll!js_Interpret(JSContext * cx=0x067edea8) Line 4939 + 0x16 bytes C++ js3250.dll!js_Invoke(JSContext * cx=0x067edea8, unsigned int argc=0x00000001, long * vp=0x061d0b98, unsigned int flags=0x00000000) Line 1323 + 0x9 bytes C++ js3250.dll!js_InternalInvoke(JSContext * cx=0x067edea8, JSObject * obj=0x06f72da0, long fval=0x06f42300, unsigned int flags=0x00000000, unsigned int argc=0x00000001, long * argv=0x067ebaf0, long * rval=0x0012f628) Line 1379 + 0x15 bytes C++ js3250.dll!JS_CallFunctionValue(JSContext * cx=0x067edea8, JSObject * obj=0x06f72da0, long fval=0x06f42300, unsigned int argc=0x00000001, long * argv=0x067ebaf0, long * rval=0x0012f628) Line 5054 + 0x1f bytes C++ gklayout.dll!nsJSContext::CallEventHandler(nsISupports * aTarget=0x06155ed8, void * aScope=0x06f72da0, void * aHandler=0x06f42300, nsIArray * aargv=0x06983bc4, nsIVariant * * arv=0x0012f6e0) Line 1962 + 0x24 bytes C++ gklayout.dll!nsGlobalWindow::RunTimeout(nsTimeout * aTimeout=0x06983c08) Line 7897 + 0xab bytes C++ gklayout.dll!nsGlobalWindow::TimerCallback(nsITimer * aTimer=0x06983c78, void * aClosure=0x06983c08) Line 8231 C++ xpcom_core.dll!nsTimerImpl::Fire() Line 400 + 0xe bytes C++ xpcom_core.dll!nsTimerEvent::Run() Line 492 C++ xpcom_core.dll!nsThread::ProcessNextEvent(int mayWait=0x00000001, int * result=0x0012f848) Line 511 C++ xpcom_core.dll!NS_ProcessNextEvent_P(nsIThread * thread=0x00bc0c40, int mayWait=0x00000001) Line 227 + 0x16 bytes C++ gkwidget.dll!nsBaseAppShell::Run() Line 170 + 0xc bytes C++ tkitcmps.dll!nsAppStartup::Run() Line 181 + 0x1c bytes C++ xul.dll!XRE_main(int argc=0x00000005, char * * argv=0x00bc8bf0, const nsXREAppData * aAppData=0x00bc19e8) Line 3180 + 0x25 bytes C++ nsCacheSession::OpenCacheEntry is probably called from the test as part of an entry presence check. The request key is: http+//localhost+8888/tests/dom/tests/mochitest/ajax/offline/simpleManifest.cacheManifest|0000001216242218|37:http://localhost:8888/tests/SimpleTest/EventUtils.js The request has no listener, is blocking, is waiting for validation, its mInfo is 0x01111104. I attach a log with enabled nsOfflineCacheUpdate:5,cache:5,nsHttp:5,mozStorage:5

And more important information: I had applied patches for complete WHATWG offline application cache support from bugs: 442806, 442812, 442813, 443023, 443017 and others related.