diff --git a/xpcom/threads/LazyIdleThread.cpp b/xpcom/threads/LazyIdleThread.cpp
index d74f515e87..c5c7a5f192 100644
--- a/xpcom/threads/LazyIdleThread.cpp
+++ b/xpcom/threads/LazyIdleThread.cpp
@@ -137,10 +137,15 @@ LazyIdleThread::EnsureThread()
     return NS_OK;
   }
 
-  MOZ_ASSERT(!mPendingEventCount, "Shouldn't have events yet!");
-  MOZ_ASSERT(!mIdleNotificationCount, "Shouldn't have idle events yet!");
-  MOZ_ASSERT(!mIdleTimer, "Should have killed this long ago!");
-  MOZ_ASSERT(!mThreadIsShuttingDown, "Should have cleared that!");
+#ifdef DEBUG
+  { // Lock scope
+    MutexAutoLock lock(mMutex);
+    MOZ_ASSERT(!mPendingEventCount, "Shouldn't have events yet!");
+    MOZ_ASSERT(!mIdleNotificationCount, "Shouldn't have idle events yet!");
+    MOZ_ASSERT(!mIdleTimer, "Should have killed this long ago!");
+    MOZ_ASSERT(!mThreadIsShuttingDown, "Should have cleared that!");
+  }
+#endif
 
   nsresult rv;
 
diff --git a/xpcom/threads/LazyIdleThread.h b/xpcom/threads/LazyIdleThread.h
index 460971c7a8..9d437171a7 100644
--- a/xpcom/threads/LazyIdleThread.h
+++ b/xpcom/threads/LazyIdleThread.h
@@ -152,9 +152,10 @@ private:
   nsCOMPtr<nsIThread> mThread;
 
   /**
-   * Protected by mMutex. Created when mThread has no pending events and fired
+   * Created when mThread has no pending events and fired
    * at mOwningThread. Any thread that dispatches to mThread will take ownership
    * of the timer and fire a separate cancel event to the owning thread.
+   * Only accessed from the owning thread.
    */
   nsCOMPtr<nsITimer> mIdleTimer;
 
diff --git a/xpcom/threads/nsProcessCommon.cpp b/xpcom/threads/nsProcessCommon.cpp
index e815ec9ea4..7d490c5952 100644
--- a/xpcom/threads/nsProcessCommon.cpp
+++ b/xpcom/threads/nsProcessCommon.cpp
@@ -280,10 +280,13 @@ nsProcess::ProcessComplete()
   }
 
   const char* topic;
-  if (mExitValue < 0) {
-    topic = "process-failed";
-  } else {
-    topic = "process-finished";
+  { // Lock scope
+    MutexAutoLock lock(mLock);
+    if (mExitValue < 0) {
+      topic = "process-failed";
+    } else {
+      topic = "process-finished";
+    }
   }
 
   mPid = -1;
@@ -412,8 +415,11 @@ nsProcess::RunProcess(bool aBlocking, char** aMyArgv, nsIObserver* aObserver,
     }
   }
 
-  mExitValue = -1;
-  mPid = -1;
+  {
+    MutexAutoLock lock(mLock);
+    mExitValue = -1;
+    mPid = -1;
+  }
 
 #if defined(PROCESSMODEL_WINAPI)
   BOOL retVal;
@@ -477,6 +483,7 @@ nsProcess::RunProcess(bool aBlocking, char** aMyArgv, nsIObserver* aObserver,
   mBlocking = aBlocking;
   if (aBlocking) {
     Monitor(this);
+    MutexAutoLock lock(mLock);
     if (mExitValue < 0) {
       return NS_ERROR_FILE_EXECUTION_FAILED;
     }