--- a/dom/media/FileBlockCache.cpp
+++ b/dom/media/FileBlockCache.cpp
@@ -98,53 +98,63 @@ FileBlockCache::FileBlockCache()
     mIsWriteScheduled(false),
     mIsOpen(false)
 {
 }
 
 FileBlockCache::~FileBlockCache()
 {
   NS_ASSERTION(!mIsOpen, "Should Close() FileBlockCache before destroying");
-  {
-    // Note, mThread will be shutdown by the time this runs, so we won't
-    // block while taking mFileMonitor.
-    MonitorAutoLock mon(mFileMonitor);
-    if (mFD) {
-      PRStatus prrc;
-      prrc = PR_Close(mFD);
-      if (prrc != PR_SUCCESS) {
-        NS_WARNING("PR_Close() failed.");
-      }
-      mFD = nullptr;
-    }
-  }
 }
 
 void FileBlockCache::Close()
 {
   LOG("Close()");
 
-  MonitorAutoLock mon(mDataMonitor);
-  if (!mIsOpen) {
-    return;
+  nsCOMPtr<nsIThread> thread;
+  {
+    MonitorAutoLock mon(mDataMonitor);
+    if (!mIsOpen) {
+      return;
+    }
+    mIsOpen = false;
+    if (!mThread) {
+      return;
+    }
+    thread.swap(mThread);
   }
-  mIsOpen = false;
-  if (!mThread) {
-    return;
+
+  PRFileDesc* fd;
+  {
+    MonitorAutoLock lock(mFileMonitor);
+    fd = mFD;
+    mFD = nullptr;
   }
-  // We must shut down the thread in another runnable. This is called
-  // while we're shutting down the media cache, and nsIThread::Shutdown()
-  // can cause events to run before it completes, which could end up
-  // opening more streams, while the media cache is shutting down and
-  // releasing memory etc! Also note we close mFD in the destructor so
-  // as to not disturb any IO that's currently running.
-  nsCOMPtr<nsIRunnable> event = new ShutdownThreadEvent(mThread);
-  SystemGroup::Dispatch(
-    "ShutdownThreadEvent", TaskCategory::Other, event.forget());
-  mThread = nullptr;
+
+  // Let the thread close the FD, and then trigger its own shutdown.
+  // Note that mThread is now empty, so no other task will be posted there.
+  // Also mThread and mFD are empty and therefore can be reused immediately.
+  nsresult rv = thread->Dispatch(NS_NewRunnableFunction([thread, fd] {
+    if (fd) {
+      PRStatus prrc;
+      prrc = PR_Close(fd);
+      if (prrc != PR_SUCCESS) {
+        NS_WARNING("PR_Close() failed.");
+      }
+    }
+    // We must shut down the thread in another runnable. This is called
+    // while we're shutting down the media cache, and nsIThread::Shutdown()
+    // can cause events to run before it completes, which could end up
+    // opening more streams, while the media cache is shutting down and
+    // releasing memory etc!
+    nsCOMPtr<nsIRunnable> event = new ShutdownThreadEvent(thread);
+    SystemGroup::Dispatch(
+      "ShutdownThreadEvent", TaskCategory::Other, event.forget());
+  }), NS_DISPATCH_NORMAL);
+  NS_ENSURE_SUCCESS_VOID(rv);
 }
 
 template<typename Container, typename Value>
 bool
 ContainerContains(const Container& aContainer, const Value& value)
 {
   return std::find(aContainer.begin(), aContainer.end(), value)
          != aContainer.end();