diff --git a/image/ProgressTracker.cpp b/image/ProgressTracker.cpp
index 8c68a691e878..ba8d2efff45b 100644
--- a/image/ProgressTracker.cpp
+++ b/image/ProgressTracker.cpp
@@ -23,39 +23,47 @@ namespace mozilla {
 namespace image {
 
 static void
-CheckProgressConsistency(Progress aProgress)
+CheckProgressConsistency(Progress aOldProgress, Progress aNewProgress)
 {
   // Check preconditions for every progress bit.
 
-  if (aProgress & FLAG_SIZE_AVAILABLE) {
+  if (aNewProgress & FLAG_SIZE_AVAILABLE) {
     // No preconditions.
   }
-  if (aProgress & FLAG_DECODE_COMPLETE) {
+  if (aNewProgress & FLAG_DECODE_COMPLETE) {
+    MOZ_ASSERT(aNewProgress & FLAG_SIZE_AVAILABLE);
+    MOZ_ASSERT(aNewProgress & (FLAG_FRAME_COMPLETE | FLAG_HAS_ERROR));
+  }
+  if (aNewProgress & FLAG_FRAME_COMPLETE) {
+    MOZ_ASSERT(aNewProgress & FLAG_SIZE_AVAILABLE);
+  }
+  if (aNewProgress & FLAG_LOAD_COMPLETE) {
+    MOZ_ASSERT(aNewProgress & (FLAG_SIZE_AVAILABLE | FLAG_HAS_ERROR));
+  }
+  if (aNewProgress & FLAG_ONLOAD_BLOCKED) {
     // No preconditions.
   }
-  if (aProgress & FLAG_FRAME_COMPLETE) {
-    // No preconditions.
+  if (aNewProgress & FLAG_ONLOAD_UNBLOCKED) {
+    MOZ_ASSERT(aNewProgress & FLAG_ONLOAD_BLOCKED);
+    MOZ_ASSERT(aNewProgress & (FLAG_SIZE_AVAILABLE | FLAG_HAS_ERROR));
   }
-  if (aProgress & FLAG_LOAD_COMPLETE) {
-    // No preconditions.
+  if (aNewProgress & FLAG_IS_ANIMATED) {
+    // No preconditions; like FLAG_HAS_TRANSPARENCY, we should normally never
+    // discover this *after* FLAG_SIZE_AVAILABLE, but unfortunately some corrupt
+    // GIFs may fool us.
   }
-  if (aProgress & FLAG_ONLOAD_BLOCKED) {
-    // No preconditions.
+  if (aNewProgress & FLAG_HAS_TRANSPARENCY) {
+    // We should never discover this after FLAG_SIZE_AVAILABLE except in the
+    // case of animated images, which may have an opaque first frame but
+    // transparent frames later on in the animation.
+    MOZ_ASSERT((aNewProgress & FLAG_IS_ANIMATED) ||
+               (aOldProgress & FLAG_HAS_TRANSPARENCY) ||
+               !(aOldProgress & FLAG_SIZE_AVAILABLE));
   }
-  if (aProgress & FLAG_ONLOAD_UNBLOCKED) {
-    MOZ_ASSERT(aProgress & FLAG_ONLOAD_BLOCKED);
-    MOZ_ASSERT(aProgress & (FLAG_SIZE_AVAILABLE | FLAG_HAS_ERROR));
+  if (aNewProgress & FLAG_LAST_PART_COMPLETE) {
+    MOZ_ASSERT(aNewProgress & FLAG_LOAD_COMPLETE);
   }
-  if (aProgress & FLAG_IS_ANIMATED) {
-    MOZ_ASSERT(aProgress & FLAG_SIZE_AVAILABLE);
-  }
-  if (aProgress & FLAG_HAS_TRANSPARENCY) {
-    MOZ_ASSERT(aProgress & FLAG_SIZE_AVAILABLE);
-  }
-  if (aProgress & FLAG_LAST_PART_COMPLETE) {
-    MOZ_ASSERT(aProgress & FLAG_LOAD_COMPLETE);
-  }
-  if (aProgress & FLAG_HAS_ERROR) {
+  if (aNewProgress & FLAG_HAS_ERROR) {
     // No preconditions.
   }
 }
@@ -371,6 +379,8 @@ ProgressTracker::SyncNotifyProgress(Progress aProgress,
     progress &= ~FLAG_ONLOAD_UNBLOCKED;
   }
 
+  CheckProgressConsistency(mProgress, mProgress | progress);
+
   // XXX(seth): Hack to work around the fact that some observers have bugs and
   // need to get onload blocking notifications multiple times. We should fix
   // those observers and remove this.
@@ -383,8 +393,6 @@ ProgressTracker::SyncNotifyProgress(Progress aProgress,
   // Apply the changes.
   mProgress |= progress;
 
-  CheckProgressConsistency(mProgress);
-
   // Send notifications.
   mObservers.Read([&](const ObserverTable* aTable) {
     SyncNotifyInternal(aTable, HasImage(), progress, aInvalidRect);
@@ -510,7 +518,6 @@ ProgressTracker::ResetForNewRequest()
 {
   MOZ_ASSERT(NS_IsMainThread());
   mProgress = NoProgress;
-  CheckProgressConsistency(mProgress);
 }
 
 void
diff --git a/image/test/crashtests/1210745-1.gif b/image/test/crashtests/1210745-1.gif
new file mode 100644
index 000000000000..92bcf7222428
Binary files /dev/null and b/image/test/crashtests/1210745-1.gif differ
diff --git a/image/test/crashtests/crashtests.list b/image/test/crashtests/crashtests.list
index cecfcedc18fe..4091d5572c5d 100644
--- a/image/test/crashtests/crashtests.list
+++ b/image/test/crashtests/crashtests.list
@@ -13,6 +13,9 @@ load 844403-1.html
 load 856616.gif
 skip-if(B2G) load 944353.jpg
 load 1205923-1.html
+# Ensure we handle detecting that an image is animated, then failing to decode
+# it. (See bug 1210745.)
+load 1210745-1.gif
 load 1212954-1.svg
 load 1235605.gif
 load 1241728-1.html