--- a/xpcom/glue/nsThreadUtils.cpp
+++ b/xpcom/glue/nsThreadUtils.cpp
@@ -12,16 +12,19 @@
 #else
 # include "nsXPCOMCIDInternal.h"
 # include "nsIThreadManager.h"
 # include "nsServiceManagerUtils.h"
 #endif
 
 #ifdef XP_WIN
 #include <windows.h>
+#include "nsWindowsHelpers.h"
+#elif defined(XP_MACOSX)
+#include <sys/resource.h>
 #endif
 
 #include <pratom.h>
 #include <prthread.h>
 
 #ifndef XPCOM_GLUE_AVOID_NSPR
 
 NS_IMPL_THREADSAFE_ISUPPORTS1(nsRunnable, nsIRunnable)
@@ -284,8 +287,41 @@ nsThreadPoolNaming::SetThreadPoolName(co
     // Set on the target thread
     NS_SetThreadName(aThread, name);
   }
   else {
     // Set on the current thread
     PR_SetCurrentThreadName(name.BeginReading());
   }
 }
+
+// nsAutoLowPriorityIO
+nsAutoLowPriorityIO::nsAutoLowPriorityIO()
+{
+#if defined(XP_WIN)
+  lowIOPrioritySet = IsVistaOrLater() &&
+                     SetThreadPriority(GetCurrentThread(),
+                                       THREAD_MODE_BACKGROUND_BEGIN);
+#elif defined(XP_MACOSX)
+  oldPriority = getiopolicy_np(IOPOL_TYPE_DISK, IOPOL_SCOPE_THREAD);
+  lowIOPrioritySet = oldPriority != -1 &&
+                     setiopolicy_np(IOPOL_TYPE_DISK,
+                                    IOPOL_SCOPE_THREAD,
+                                    IOPOL_THROTTLE) != -1;
+#else
+  lowIOPrioritySet = false;
+#endif
+}
+
+nsAutoLowPriorityIO::~nsAutoLowPriorityIO()
+{
+#if defined(XP_WIN)
+  if (NS_LIKELY(lowIOPrioritySet)) {
+    // On Windows the old thread priority is automatically restored
+    SetThreadPriority(GetCurrentThread(), THREAD_MODE_BACKGROUND_END);
+  }
+#elif defined(XP_MACOSX)
+  if (NS_LIKELY(lowIOPrioritySet)) {
+    setiopolicy_np(IOPOL_TYPE_DISK, IOPOL_SCOPE_THREAD, oldPriority);
+  }
+#endif
+}
+

--- a/xpcom/glue/nsThreadUtils.h
+++ b/xpcom/glue/nsThreadUtils.h
@@ -489,9 +489,30 @@ public:
 
 private:
   volatile PRUint32 mCounter;
 
   nsThreadPoolNaming(const nsThreadPoolNaming &) MOZ_DELETE;
   void operator=(const nsThreadPoolNaming &) MOZ_DELETE;
 };
 
+/**
+ * Thread priority in most operating systems affect scheduling, not IO.  This
+ * helper is used to set the current thread to low IO priority for the lifetime
+ * of the created object.  You can only use this low priority IO setting within
+ * the context of the current thread.
+ */
+class NS_STACK_CLASS nsAutoLowPriorityIO
+{
+public:
+  nsAutoLowPriorityIO();
+  ~nsAutoLowPriorityIO();
+
+private:
+  bool lowIOPrioritySet;
+#if defined(XP_MACOSX)
+  int oldPriority;
+#endif
+};
+
+
+
 #endif  // nsThreadUtils_h__