diff --git a/image/public/imgIContainer.idl b/image/public/imgIContainer.idl
index 23f7b7c6771..f5101661937 100644
--- a/image/public/imgIContainer.idl
+++ b/image/public/imgIContainer.idl
@@ -69,7 +69,7 @@ native nsIntSizeByVal(nsIntSize);
  *
  * Internally, imgIContainer also manages animation of images.
  */
-[scriptable, builtinclass, uuid(25236a61-9568-4b2b-a2ce-209a2db9771d)]
+[scriptable, builtinclass, uuid(60e07aae-8500-46c7-b305-9a35ad7e6665)]
 interface imgIContainer : nsISupports
 {
   /**
@@ -151,6 +151,9 @@ interface imgIContainer : nsISupports
    * FLAG_SYNC_DECODE: Forces synchronous/non-progressive decode of all
    * available data before the call returns.
    *
+   * FLAG_SYNC_DECODE_IF_FAST: Like FLAG_SYNC_DECODE, but requests a sync decode
+   * be performed only if ImageLib estimates it can be completed very quickly.
+   *
    * FLAG_DECODE_NO_PREMULTIPLY_ALPHA: Do not premultiply alpha if
    * it's not already premultiplied in the image data.
    *
@@ -177,12 +180,13 @@ interface imgIContainer : nsISupports
    */
   const unsigned long FLAG_NONE                            = 0x0;
   const unsigned long FLAG_SYNC_DECODE                     = 0x1;
-  const unsigned long FLAG_DECODE_NO_PREMULTIPLY_ALPHA     = 0x2;
-  const unsigned long FLAG_DECODE_NO_COLORSPACE_CONVERSION = 0x4;
-  const unsigned long FLAG_CLAMP                           = 0x8;
-  const unsigned long FLAG_HIGH_QUALITY_SCALING            = 0x10;
-  const unsigned long FLAG_WANT_DATA_SURFACE               = 0x20;
-  const unsigned long FLAG_BYPASS_SURFACE_CACHE            = 0x40;
+  const unsigned long FLAG_SYNC_DECODE_IF_FAST             = 0x2;
+  const unsigned long FLAG_DECODE_NO_PREMULTIPLY_ALPHA     = 0x4;
+  const unsigned long FLAG_DECODE_NO_COLORSPACE_CONVERSION = 0x8;
+  const unsigned long FLAG_CLAMP                           = 0x10;
+  const unsigned long FLAG_HIGH_QUALITY_SCALING            = 0x20;
+  const unsigned long FLAG_WANT_DATA_SURFACE               = 0x40;
+  const unsigned long FLAG_BYPASS_SURFACE_CACHE            = 0x80;
 
   /**
    * A constant specifying the default set of decode flags (i.e., the default
diff --git a/image/src/RasterImage.cpp b/image/src/RasterImage.cpp
index ae01a97ebfb..a56b49d43ef 100644
--- a/image/src/RasterImage.cpp
+++ b/image/src/RasterImage.cpp
@@ -1420,23 +1420,18 @@ RasterImage::WantDecodedFrames(const nsIntSize& aSize, uint32_t aFlags,
     SurfaceCache::UnlockSurfaces(ImageKey(this));
   }
 
+  DecodeStrategy strategy = DecodeStrategy::ASYNC;
+
   if (aShouldSyncNotify) {
     // We can sync notify, which means we can also sync decode.
     if (aFlags & FLAG_SYNC_DECODE) {
-      Decode(DecodeStrategy::SYNC_IF_POSSIBLE, Some(aSize), aFlags);
-      return;
+      strategy = DecodeStrategy::SYNC_IF_POSSIBLE;
+    } else if (aFlags & FLAG_SYNC_DECODE_IF_FAST) {
+      strategy = DecodeStrategy::SYNC_FOR_SMALL_IMAGES;
     }
-
-    // Here we are explicitly trading off flashing for responsiveness in the
-    // case that we're redecoding an image (see bug 845147).
-    Decode(mHasBeenDecoded ? DecodeStrategy::ASYNC
-                           : DecodeStrategy::SYNC_FOR_SMALL_IMAGES,
-           Some(aSize), aFlags);
-    return;
   }
 
-  // We can't sync notify, so do an async decode.
-  Decode(DecodeStrategy::ASYNC, Some(aSize), aFlags);
+  Decode(strategy, Some(aSize), aFlags);
 }
 
 //******************************************************************************
@@ -1466,7 +1461,7 @@ RasterImage::StartDecoding()
     return NS_OK;
   }
 
-  return RequestDecodeForSize(mSize, FLAG_SYNC_DECODE);
+  return RequestDecodeForSize(mSize, FLAG_SYNC_DECODE_IF_FAST);
 }
 
 NS_IMETHODIMP
@@ -1487,8 +1482,15 @@ RasterImage::RequestDecodeForSize(const nsIntSize& aSize, uint32_t aFlags)
   // downscale-during-decode.
   nsIntSize targetSize = mDownscaleDuringDecode ? aSize : mSize;
 
-  // Sync decode small images if requested.
-  bool shouldSyncDecodeSmallImages = aFlags & FLAG_SYNC_DECODE;
+  // Decide whether to sync decode images we can decode quickly. Here we are
+  // explicitly trading off flashing for responsiveness in the case that we're
+  // redecoding an image (see bug 845147).
+  bool shouldSyncDecodeIfFast =
+    !mHasBeenDecoded && (aFlags & FLAG_SYNC_DECODE_IF_FAST);
+
+  uint32_t flags = shouldSyncDecodeIfFast
+                 ? aFlags
+                 : aFlags & ~FLAG_SYNC_DECODE_IF_FAST;
 
   // Look up the first frame of the image, which will implicitly start decoding
   // if it's not available right now.
@@ -1496,8 +1498,8 @@ RasterImage::RequestDecodeForSize(const nsIntSize& aSize, uint32_t aFlags)
   // synchronously decoding small images, while passing false has the effect of
   // decoding asynchronously, but that's not obvious from the argument name.
   // This API needs to be reworked.
-  LookupFrame(0, targetSize, DecodeFlags(aFlags),
-              /* aShouldSyncNotify = */ shouldSyncDecodeSmallImages);
+  LookupFrame(0, targetSize, flags,
+              /* aShouldSyncNotify = */ shouldSyncDecodeIfFast);
 
   return NS_OK;
 }