--- a/image/src/ProgressTracker.cpp
+++ b/image/src/ProgressTracker.cpp
@@ -58,38 +58,29 @@ CheckProgressConsistency(Progress aProgr
   }
   if (aProgress & FLAG_FRAME_COMPLETE) {
     MOZ_ASSERT(aProgress & FLAG_DECODE_STARTED);
   }
   if (aProgress & FLAG_LOAD_COMPLETE) {
     // No preconditions.
   }
   if (aProgress & FLAG_ONLOAD_BLOCKED) {
-    if (aProgress & FLAG_IS_MULTIPART) {
-      MOZ_ASSERT(aProgress & FLAG_ONLOAD_UNBLOCKED);
-    } else {
-      MOZ_ASSERT(aProgress & FLAG_DECODE_STARTED);
-    }
+    MOZ_ASSERT(aProgress & FLAG_DECODE_STARTED);
   }
   if (aProgress & FLAG_ONLOAD_UNBLOCKED) {
     MOZ_ASSERT(aProgress & FLAG_ONLOAD_BLOCKED);
-    MOZ_ASSERT(aProgress & (FLAG_FRAME_COMPLETE |
-                            FLAG_IS_MULTIPART |
-                            FLAG_HAS_ERROR));
+    MOZ_ASSERT(aProgress & (FLAG_FRAME_COMPLETE | FLAG_HAS_ERROR));
   }
   if (aProgress & FLAG_IS_ANIMATED) {
     MOZ_ASSERT(aProgress & FLAG_DECODE_STARTED);
     MOZ_ASSERT(aProgress & FLAG_SIZE_AVAILABLE);
   }
   if (aProgress & FLAG_HAS_TRANSPARENCY) {
     MOZ_ASSERT(aProgress & FLAG_SIZE_AVAILABLE);
   }
-  if (aProgress & FLAG_IS_MULTIPART) {
-    // No preconditions.
-  }
   if (aProgress & FLAG_LAST_PART_COMPLETE) {
     MOZ_ASSERT(aProgress & FLAG_LOAD_COMPLETE);
   }
   if (aProgress & FLAG_HAS_ERROR) {
     // No preconditions.
   }
 }
 
@@ -103,32 +94,16 @@ ProgressTracker::SetImage(Image* aImage)
 
 void
 ProgressTracker::ResetImage()
 {
   NS_ABORT_IF_FALSE(mImage, "Resetting image when it's already null!");
   mImage = nullptr;
 }
 
-void
-ProgressTracker::SetIsMultipart()
-{
-  if (mProgress & FLAG_IS_MULTIPART) {
-    return;
-  }
-
-  MOZ_ASSERT(!(mProgress & FLAG_ONLOAD_BLOCKED),
-             "Blocked onload before we knew we were multipart?");
-
-  // Set the MULTIPART flag and ensure that we never block onload.
-  mProgress |= FLAG_IS_MULTIPART | FLAG_ONLOAD_BLOCKED | FLAG_ONLOAD_UNBLOCKED;
-
-  CheckProgressConsistency(mProgress);
-}
-
 bool
 ProgressTracker::IsLoading() const
 {
   // Checking for whether OnStopRequest has fired allows us to say we're
   // loading before OnStartRequest gets called, letting the request properly
   // get removed from the cache in certain cases.
   return !(mProgress & FLAG_LOAD_COMPLETE);
 }

--- a/image/src/ProgressTracker.h
+++ b/image/src/ProgressTracker.h
@@ -31,19 +31,18 @@ enum {
   FLAG_DECODE_STARTED     = 1u << 1,  // STATUS_DECODE_STARTED
   FLAG_DECODE_COMPLETE    = 1u << 2,  // STATUS_DECODE_COMPLETE
   FLAG_FRAME_COMPLETE     = 1u << 3,  // STATUS_FRAME_COMPLETE
   FLAG_LOAD_COMPLETE      = 1u << 4,  // STATUS_LOAD_COMPLETE
   FLAG_ONLOAD_BLOCKED     = 1u << 5,
   FLAG_ONLOAD_UNBLOCKED   = 1u << 6,
   FLAG_IS_ANIMATED        = 1u << 7,  // STATUS_IS_ANIMATED
   FLAG_HAS_TRANSPARENCY   = 1u << 8,  // STATUS_HAS_TRANSPARENCY
-  FLAG_IS_MULTIPART       = 1u << 9,
-  FLAG_LAST_PART_COMPLETE = 1u << 10,
-  FLAG_HAS_ERROR          = 1u << 11  // STATUS_ERROR
+  FLAG_LAST_PART_COMPLETE = 1u << 9,
+  FLAG_HAS_ERROR          = 1u << 10  // STATUS_ERROR
 };
 
 typedef uint32_t Progress;
 
 const uint32_t NoProgress = 0;
 
 inline Progress LoadCompleteProgress(bool aLastPart,
                                      bool aError,
@@ -84,19 +83,16 @@ public:
 
   bool HasImage() const { return mImage; }
   already_AddRefed<Image> GetImage() const
   {
     nsRefPtr<Image> image = mImage;
     return image.forget();
   }
 
-  // Informs this ProgressTracker that it's associated with a multipart image.
-  void SetIsMultipart();
-
   // Returns whether we are in the process of loading; that is, whether we have
   // not received OnStopRequest from Necko.
   bool IsLoading() const;
 
   // Get the current image status (as in imgIRequest).
   uint32_t GetImageStatus() const;
 
   // Get the current Progress.
@@ -162,22 +158,16 @@ public:
     return mObservers.Length();
   }
 
   // This is intentionally non-general because its sole purpose is to support
   // some obscure network priority logic in imgRequest. That stuff could
   // probably be improved, but it's too scary to mess with at the moment.
   bool FirstObserverIs(IProgressObserver* aObserver);
 
-  void AdoptObservers(ProgressTracker* aTracker) {
-    MOZ_ASSERT(NS_IsMainThread(), "Use mObservers on main thread only");
-    MOZ_ASSERT(aTracker);
-    mObservers = aTracker->mObservers;
-  }
-
 private:
   typedef nsTObserverArray<mozilla::WeakPtr<IProgressObserver>> ObserverArray;
   friend class AsyncNotifyRunnable;
   friend class AsyncNotifyCurrentStateRunnable;
   friend class ProgressTrackerInit;
 
   ProgressTracker(const ProgressTracker& aOther) MOZ_DELETE;