--- a/netwerk/base/ProxyAutoConfig.cpp
+++ b/netwerk/base/ProxyAutoConfig.cpp
@@ -867,28 +867,30 @@ ProxyAutoConfig::GC()
 
   JSAutoCompartment ac(mJSContext->Context(), mJSContext->Global());
   JS_MaybeGC(mJSContext->Context());
 }
 
 ProxyAutoConfig::~ProxyAutoConfig()
 {
   MOZ_COUNT_DTOR(ProxyAutoConfig);
+  MOZ_ASSERT(mShutdown, "Shutdown must be called before dtor.");
   NS_ASSERTION(!mJSContext,
                "~ProxyAutoConfig leaking JS context that "
                "should have been deleted on pac thread");
 }
 
 void
 ProxyAutoConfig::Shutdown()
 {
   MOZ_ASSERT(!NS_IsMainThread(), "wrong thread for shutdown");
 
-  if (GetRunning() || mShutdown)
+  if (NS_WARN_IF(GetRunning()) || mShutdown) {
     return;
+  }
 
   mShutdown = true;
   delete mJSContext;
   mJSContext = nullptr;
 }
 
 bool
 ProxyAutoConfig::SrcAddress(const NetAddr *remoteAddress, nsCString &localAddress)

--- a/netwerk/base/nsPACMan.cpp
+++ b/netwerk/base/nsPACMan.cpp
@@ -343,16 +343,18 @@ nsPACMan::nsPACMan(nsIEventTarget *mainT
     PR_NewThreadPrivateIndex(&sThreadLocalIndex, nullptr);
   }
   mPAC.SetThreadLocalIndex(sThreadLocalIndex);
   mIncludePath = Preferences::GetBool(kPACIncludePath, false);
 }
 
 nsPACMan::~nsPACMan()
 {
+  MOZ_ASSERT(mShutdown, "Shutdown must be called before dtor.");
+
   if (mPACThread) {
     if (NS_IsMainThread()) {
       mPACThread->Shutdown();
     }
     else {
       RefPtr<ShutdownThread> runnable = new ShutdownThread(mPACThread);
       Dispatch(runnable.forget());
     }
@@ -366,16 +368,18 @@ void
 nsPACMan::Shutdown()
 {
   MOZ_ASSERT(NS_IsMainThread(), "pacman must be shutdown on main thread");
   if (mShutdown) {
     return;
   }
   mShutdown = true;
   CancelExistingLoad();
+
+  MOZ_ASSERT(mPACThread, "mPAC requires mPACThread to shutdown");
   PostCancelPendingQ(NS_ERROR_ABORT);
 
   RefPtr<WaitForThreadShutdown> runnable = new WaitForThreadShutdown(this);
   Dispatch(runnable.forget());
 }
 
 nsresult
 nsPACMan::AsyncGetProxyForURI(nsIURI *uri,