--- a/image/src/DiscardTracker.cpp
+++ b/image/src/DiscardTracker.cpp
@@ -14,17 +14,17 @@ namespace image {
 
 static const char* sDiscardTimeoutPref = "image.mem.min_discard_timeout_ms";
 
 /* static */ LinkedList<DiscardTracker::Node> DiscardTracker::sDiscardableImages;
 /* static */ nsCOMPtr<nsITimer> DiscardTracker::sTimer;
 /* static */ bool DiscardTracker::sInitialized = false;
 /* static */ bool DiscardTracker::sTimerOn = false;
 /* static */ bool DiscardTracker::sDiscardRunnablePending = false;
-/* static */ ssize_t DiscardTracker::sCurrentDecodedImageBytes = 0;
+/* static */ PRUint64 DiscardTracker::sCurrentDecodedImageBytes = 0;
 /* static */ PRUint32 DiscardTracker::sMinDiscardTimeoutMs = 10000;
 /* static */ PRUint32 DiscardTracker::sMaxDecodedImageKB = 42 * 1024;
 
 /*
  * When we notice we're using too much memory for decoded images, we enqueue a
  * DiscardRunnable, which runs this code.
  */
 NS_IMETHODIMP
@@ -114,17 +114,17 @@ DiscardTracker::DiscardAll()
 
   sDiscardableImages.clear();
 
   // The list is empty, so there's no need to leave the timer on.
   DisableTimer();
 }
 
 void
-DiscardTracker::InformAllocation(ssize_t bytes)
+DiscardTracker::InformAllocation(PRUint64 bytes)
 {
   // This function is called back e.g. from RasterImage::Discard(); be careful!
 
   sCurrentDecodedImageBytes += bytes;
   MOZ_ASSERT(sCurrentDecodedImageBytes >= 0);
 
   // If we're using too much memory for decoded images, MaybeDiscardSoon will
   // enqueue a callback to discard some images.

--- a/image/src/DiscardTracker.h
+++ b/image/src/DiscardTracker.h
@@ -70,17 +70,17 @@ class DiscardTracker
      */
     static void DiscardAll();
 
     /**
      * Inform the discard tracker that we've allocated or deallocated some
      * memory for a decoded image.  We use this to determine when we've
      * allocated too much memory and should discard some images.
      */
-    static void InformAllocation(ssize_t bytes);
+    static void InformAllocation(PRUint64 bytes);
 
   private:
     /**
      * This is called when the discard timer fires; it calls into DiscardNow().
      */
     friend int DiscardTimeoutChangedCallback(const char* aPref, void *aClosure);
 
     /**
@@ -100,17 +100,17 @@ class DiscardTracker
     static void TimerCallback(nsITimer *aTimer, void *aClosure);
     static void DiscardNow();
 
     static LinkedList<Node> sDiscardableImages;
     static nsCOMPtr<nsITimer> sTimer;
     static bool sInitialized;
     static bool sTimerOn;
     static bool sDiscardRunnablePending;
-    static ssize_t sCurrentDecodedImageBytes;
+    static PRUint64 sCurrentDecodedImageBytes;
     static PRUint32 sMinDiscardTimeoutMs;
     static PRUint32 sMaxDecodedImageKB;
 };
 
 } // namespace image
 } // namespace mozilla
 
 #endif /* mozilla_imagelib_DiscardTracker_h_ */