--- a/js/xpconnect/src/XPCJSContext.cpp
+++ b/js/xpconnect/src/XPCJSContext.cpp
@@ -995,22 +995,23 @@ class Watchdog
         {   // Scoped lock.
             AutoLockWatchdog lock(this);
 
             // Signal to the watchdog thread that it's time to shut down.
             mShuttingDown = true;
 
             // Wake up the watchdog, and wait for it to call us back.
             PR_NotifyCondVar(mWakeup);
-            PR_WaitCondVar(mWakeup, PR_INTERVAL_NO_TIMEOUT);
-            MOZ_ASSERT(!mShuttingDown);
         }
 
         PR_JoinThread(mThread);
 
+        // The thread sets mShuttingDown to false as it exits.
+        MOZ_ASSERT(!mShuttingDown);
+
         // Destroy state.
         mThread = nullptr;
         PR_DestroyCondVar(mWakeup);
         mWakeup = nullptr;
         PR_DestroyLock(mLock);
         mLock = nullptr;
 
         // All done.
@@ -1039,17 +1040,16 @@ class Watchdog
     {
         MOZ_ASSERT(!NS_IsMainThread());
         MOZ_ALWAYS_TRUE(PR_WaitCondVar(mWakeup, timeout) == PR_SUCCESS);
     }
     void Finished()
     {
         MOZ_ASSERT(!NS_IsMainThread());
         mShuttingDown = false;
-        PR_NotifyCondVar(mWakeup);
     }
 
     int32_t MinScriptRunTimeSeconds()
     {
         return mMinScriptRunTimeSeconds;
     }
 
   private: