--- a/image/decoders/nsPNGDecoder.cpp
+++ b/image/decoders/nsPNGDecoder.cpp
@@ -164,17 +164,16 @@ void nsPNGDecoder::CreateFrame(png_uint_
     }
 
     NeedNewFrame(mNumFrames, x_offset, y_offset, width, height, format);
   } else if (mNumFrames != 0) {
     NeedNewFrame(mNumFrames, x_offset, y_offset, width, height, format);
   }
 
   mFrameRect = neededRect;
-  mFrameHasNoAlpha = true;
 
   PR_LOG(GetPNGDecoderAccountingLog(), PR_LOG_DEBUG,
          ("PNGDecoderAccounting: nsPNGDecoder::CreateFrame -- created "
           "image frame with %dx%d pixels in container %p",
           width, height,
           &mImage));
 
 #ifdef PNG_APNG_SUPPORTED
@@ -196,17 +195,17 @@ nsPNGDecoder::EndImageFrame()
 {
   if (mFrameIsHidden) {
     return;
   }
 
   mNumFrames++;
 
   Opacity opacity = Opacity::SOME_TRANSPARENCY;
-  if (format == gfx::SurfaceFormat::B8G8R8X8 || mFrameHasNoAlpha) {
+  if (format == gfx::SurfaceFormat::B8G8R8X8) {
     opacity = Opacity::OPAQUE;
   }
 
 #ifdef PNG_APNG_SUPPORTED
   uint32_t numFrames = GetFrameCount();
 
   // We can't use mPNG->num_frames_read as it may be one ahead.
   if (numFrames > 1) {
@@ -734,17 +733,16 @@ nsPNGDecoder::row_callback(png_structp p
     png_bytep line = new_row;
     if (decoder->interlacebuf) {
       line = decoder->interlacebuf + (row_num * decoder->mChannels * width);
       png_progressive_combine_row(png_ptr, line, new_row);
     }
 
     uint32_t bpr = width * sizeof(uint32_t);
     uint32_t* cptr32 = (uint32_t*)(decoder->mImageData + (row_num*bpr));
-    bool rowHasNoAlpha = true;
 
     if (decoder->mTransform) {
       if (decoder->mCMSLine) {
         qcms_transform_data(decoder->mTransform, line, decoder->mCMSLine,
                             iwidth);
         // copy alpha over
         uint32_t channels = decoder->mChannels;
         if (channels == 2 || channels == 4) {
@@ -783,41 +781,31 @@ nsPNGDecoder::row_callback(png_structp p
           line += 3;
         }
       }
       break;
       case gfx::SurfaceFormat::B8G8R8A8: {
         if (!decoder->mDisablePremultipliedAlpha) {
           for (uint32_t x=width; x>0; --x) {
             *cptr32++ = gfxPackedPixel(line[3], line[0], line[1], line[2]);
-            if (line[3] != 0xff) {
-              rowHasNoAlpha = false;
-            }
             line += 4;
           }
         } else {
           for (uint32_t x=width; x>0; --x) {
             *cptr32++ = gfxPackedPixelNoPreMultiply(line[3], line[0], line[1],
                                                     line[2]);
-            if (line[3] != 0xff) {
-              rowHasNoAlpha = false;
-            }
             line += 4;
           }
         }
       }
       break;
       default:
         png_longjmp(decoder->mPNG, 1);
     }
 
-    if (!rowHasNoAlpha) {
-      decoder->mFrameHasNoAlpha = false;
-    }
-
     if (decoder->mNumFrames <= 1) {
       // Only do incremental image display for the first frame
       // XXXbholley - this check should be handled in the superclass
       nsIntRect r(0, row_num, width, 1);
       decoder->PostInvalidation(r);
     }
   }
 }

--- a/image/decoders/nsPNGDecoder.h
+++ b/image/decoders/nsPNGDecoder.h
@@ -81,17 +81,16 @@ public:
   // For size decodes
   uint8_t mSizeBytes[8]; // Space for width and height, both 4 bytes
   uint32_t mHeaderBytesRead;
 
   // whether CMS or premultiplied alpha are forced off
   uint32_t mCMSMode;
 
   uint8_t mChannels;
-  bool mFrameHasNoAlpha;
   bool mFrameIsHidden;
   bool mDisablePremultipliedAlpha;
 
   struct AnimFrameInfo
   {
     AnimFrameInfo();
 #ifdef PNG_APNG_SUPPORTED
     AnimFrameInfo(png_structp aPNG, png_infop aInfo);