diff --git a/image/IDecodingTask.cpp b/image/IDecodingTask.cpp
index d92aca7cb778..8c598bf33a62 100644
--- a/image/IDecodingTask.cpp
+++ b/image/IDecodingTask.cpp
@@ -13,6 +13,8 @@
 #include "RasterImage.h"
 #include "SurfaceCache.h"
 
+#include "mozilla/SystemGroup.h"
+
 namespace mozilla {
 
 using gfx::IntRect;
@@ -23,11 +25,47 @@ namespace image {
 // Helpers for sending notifications to the image associated with a decoder.
 ///////////////////////////////////////////////////////////////////////////////
 
-/* static */ void
+void
+IDecodingTask::EnsureHasEventTarget(NotNull<RasterImage*> aImage)
+{
+  if (!mEventTarget) {
+    // We determine the event target as late as possible, at the first dispatch
+    // time, because the observers bound to an imgRequest will affect it.
+    // We cache it rather than query for the event target each time because the
+    // event target can change. We don't want to risk events being executed in
+    // a different order than they are dispatched, which can happen if we
+    // selected scheduler groups which have no ordering guarantees relative to
+    // each other (e.g. it moves from scheduler group A for doc group DA to
+    // scheduler group B for doc group DB due to changing observers -- if we
+    // dispatched the first event on A, and the second on B, we don't know which
+    // will execute first.)
+    RefPtr<ProgressTracker> tracker = aImage->GetProgressTracker();
+    if (tracker) {
+      mEventTarget = tracker->GetEventTarget();
+    } else {
+      mEventTarget = SystemGroup::EventTargetFor(TaskCategory::Other);
+    }
+  }
+}
+
+bool
+IDecodingTask::IsOnEventTarget() const
+{
+  // This is essentially equivalent to NS_IsOnMainThread() because all of the
+  // event targets are for the main thread (although perhaps with a different
+  // label / scheduler group). The observers in ProgressTracker may have
+  // different event targets from this, so this is just a best effort guess.
+  bool current = false;
+  mEventTarget->IsOnCurrentThread(&current);
+  return current;
+}
+
+void
 IDecodingTask::NotifyProgress(NotNull<RasterImage*> aImage,
                               NotNull<Decoder*> aDecoder)
 {
   MOZ_ASSERT(aDecoder->HasProgress() && !aDecoder->IsMetadataDecode());
+  EnsureHasEventTarget(aImage);
 
   // Capture the decoder's state. If we need to notify asynchronously, it's
   // important that we don't wait until the lambda actually runs to capture the
@@ -42,7 +80,7 @@ IDecodingTask::NotifyProgress(NotNull<RasterImage*> aImage,
   SurfaceFlags surfaceFlags = aDecoder->GetSurfaceFlags();
 
   // Synchronously notify if we can.
-  if (NS_IsMainThread() && !(decoderFlags & DecoderFlags::ASYNC_NOTIFY)) {
+  if (IsOnEventTarget() && !(decoderFlags & DecoderFlags::ASYNC_NOTIFY)) {
     aImage->NotifyProgress(progress, invalidRect, frameCount,
                            decoderFlags, surfaceFlags);
     return;
@@ -50,20 +88,21 @@ IDecodingTask::NotifyProgress(NotNull<RasterImage*> aImage,
 
   // We're forced to notify asynchronously.
   NotNull<RefPtr<RasterImage>> image = aImage;
-  NS_DispatchToMainThread(NS_NewRunnableFunction(
-                            "IDecodingTask::NotifyProgress",
-                            [=]() -> void {
+  mEventTarget->Dispatch(NS_NewRunnableFunction(
+                           "IDecodingTask::NotifyProgress",
+                           [=]() -> void {
     image->NotifyProgress(progress, invalidRect, frameCount,
                           decoderFlags, surfaceFlags);
-  }));
+  }), NS_DISPATCH_NORMAL);
 }
 
-/* static */ void
+void
 IDecodingTask::NotifyDecodeComplete(NotNull<RasterImage*> aImage,
                                     NotNull<Decoder*> aDecoder)
 {
   MOZ_ASSERT(aDecoder->HasError() || !aDecoder->InFrame(),
              "Decode complete in the middle of a frame?");
+  EnsureHasEventTarget(aImage);
 
   // Capture the decoder's state.
   DecoderFinalStatus finalStatus = aDecoder->FinalStatus();
@@ -76,7 +115,7 @@ IDecodingTask::NotifyDecodeComplete(NotNull<RasterImage*> aImage,
   SurfaceFlags surfaceFlags = aDecoder->GetSurfaceFlags();
 
   // Synchronously notify if we can.
-  if (NS_IsMainThread() && !(decoderFlags & DecoderFlags::ASYNC_NOTIFY)) {
+  if (IsOnEventTarget() && !(decoderFlags & DecoderFlags::ASYNC_NOTIFY)) {
     aImage->NotifyDecodeComplete(finalStatus, metadata, telemetry, progress,
                                  invalidRect, frameCount, decoderFlags,
                                  surfaceFlags);
@@ -85,13 +124,13 @@ IDecodingTask::NotifyDecodeComplete(NotNull<RasterImage*> aImage,
 
   // We're forced to notify asynchronously.
   NotNull<RefPtr<RasterImage>> image = aImage;
-  NS_DispatchToMainThread(NS_NewRunnableFunction(
-                            "IDecodingTask::NotifyDecodeComplete",
-                            [=]() -> void {
+  mEventTarget->Dispatch(NS_NewRunnableFunction(
+                           "IDecodingTask::NotifyDecodeComplete",
+                           [=]() -> void {
     image->NotifyDecodeComplete(finalStatus, metadata, telemetry, progress,
                                 invalidRect, frameCount, decoderFlags,
                                 surfaceFlags);
-  }));
+  }), NS_DISPATCH_NORMAL);
 }
 
 
diff --git a/image/IDecodingTask.h b/image/IDecodingTask.h
index 196ce5fdc44a..cf8daa462cad 100644
--- a/image/IDecodingTask.h
+++ b/image/IDecodingTask.h
@@ -50,15 +50,22 @@ public:
   void Resume() override;
 
 protected:
+  virtual ~IDecodingTask() { }
+
   /// Notify @aImage of @aDecoder's progress.
-  static void NotifyProgress(NotNull<RasterImage*> aImage,
-                             NotNull<Decoder*> aDecoder);
+  void NotifyProgress(NotNull<RasterImage*> aImage,
+                      NotNull<Decoder*> aDecoder);
 
   /// Notify @aImage that @aDecoder has finished.
-  static void NotifyDecodeComplete(NotNull<RasterImage*> aImage,
-                                   NotNull<Decoder*> aDecoder);
+  void NotifyDecodeComplete(NotNull<RasterImage*> aImage,
+                            NotNull<Decoder*> aDecoder);
 
-  virtual ~IDecodingTask() { }
+private:
+  void EnsureHasEventTarget(NotNull<RasterImage*> aImage);
+
+  bool IsOnEventTarget() const;
+
+  nsCOMPtr<nsIEventTarget> mEventTarget;
 };