diff --git a/image/FrameAnimator.cpp b/image/FrameAnimator.cpp
index f1d8faea740..df12787f764 100644
--- a/image/FrameAnimator.cpp
+++ b/image/FrameAnimator.cpp
@@ -439,7 +439,7 @@ FrameAnimator::DoBlend(nsIntRect* aDirtyRect,
   // Calculate area that needs updating
   switch (prevFrameData.mDisposalMethod) {
     default:
-      MOZ_ASSERT_UNREACHABLE("Unexpected DisposalMethod");
+      MOZ_FALLTHROUGH_ASSERT("Unexpected DisposalMethod");
     case DisposalMethod::NOT_SPECIFIED:
     case DisposalMethod::KEEP:
       *aDirtyRect = nextFrameData.mRect;
@@ -573,7 +573,7 @@ FrameAnimator::DoBlend(nsIntRect* aDirtyRect,
         break;
 
       default:
-        MOZ_ASSERT_UNREACHABLE("Unexpected DisposalMethod");
+        MOZ_FALLTHROUGH_ASSERT("Unexpected DisposalMethod");
       case DisposalMethod::NOT_SPECIFIED:
       case DisposalMethod::KEEP:
         // Copy previous frame into compositingFrame before we put the new
diff --git a/image/decoders/nsGIFDecoder2.cpp b/image/decoders/nsGIFDecoder2.cpp
index b100a08d554..91de60156b0 100644
--- a/image/decoders/nsGIFDecoder2.cpp
+++ b/image/decoders/nsGIFDecoder2.cpp
@@ -1145,8 +1145,7 @@ nsGIFDecoder2::WriteInternal(const char* aBuffer, uint32_t aCount)
         mGIFStruct.state = gif_error;
         return;
       }
-
-      // FALL THROUGH
+      MOZ_FALLTHROUGH; // to continue decoding header.
     }
 
     case gif_image_header_continue: {
diff --git a/image/decoders/nsJPEGDecoder.cpp b/image/decoders/nsJPEGDecoder.cpp
index d8fdc255817..56c35505e7c 100644
--- a/image/decoders/nsJPEGDecoder.cpp
+++ b/image/decoders/nsJPEGDecoder.cpp
@@ -397,6 +397,7 @@ nsJPEGDecoder::WriteInternal(const char* aBuffer, uint32_t aCount)
             mInfo.output_width, mInfo.output_height));
 
     mState = JPEG_START_DECOMPRESS;
+    MOZ_FALLTHROUGH; // to start decompressing.
   }
 
   case JPEG_START_DECOMPRESS: {
@@ -423,6 +424,7 @@ nsJPEGDecoder::WriteInternal(const char* aBuffer, uint32_t aCount)
     // If this is a progressive JPEG ...
     mState = mInfo.buffered_image ?
              JPEG_DECOMPRESS_PROGRESSIVE : JPEG_DECOMPRESS_SEQUENTIAL;
+    MOZ_FALLTHROUGH; // to decompress sequential JPEG.
   }
 
   case JPEG_DECOMPRESS_SEQUENTIAL: {
@@ -444,6 +446,7 @@ nsJPEGDecoder::WriteInternal(const char* aBuffer, uint32_t aCount)
                    "We didn't process all of the data!");
       mState = JPEG_DONE;
     }
+    MOZ_FALLTHROUGH; // to decompress progressive JPEG.
   }
 
   case JPEG_DECOMPRESS_PROGRESSIVE: {
@@ -516,6 +519,7 @@ nsJPEGDecoder::WriteInternal(const char* aBuffer, uint32_t aCount)
 
       mState = JPEG_DONE;
     }
+    MOZ_FALLTHROUGH; // to finish decompressing.
   }
 
   case JPEG_DONE: {