diff --git a/image/encoders/bmp/nsBMPEncoder.cpp b/image/encoders/bmp/nsBMPEncoder.cpp
index ade37621afc..fe1f84bebb6 100644
--- a/image/encoders/bmp/nsBMPEncoder.cpp
+++ b/image/encoders/bmp/nsBMPEncoder.cpp
@@ -437,17 +437,26 @@ void
 nsBMPEncoder::ConvertHostARGBRow(const PRUint8* aSrc, PRUint8* aDest,
                                  PRUint32 aPixelWidth)
 {
-  for (PRUint32 x = 0; x < aPixelWidth; x ++) {
-    const PRUint32& pixelIn = ((const PRUint32*)(aSrc))[x];
-    PRUint8 *pixelOut = &aDest[x * BytesPerPixel(mBMPInfoHeader.bpp)];
+  int bytes = BytesPerPixel(mBMPInfoHeader.bpp);
 
-    PRUint8 alpha = (pixelIn & 0xff000000) >> 24;
-    pixelOut[0] = (((pixelIn & 0xff0000) >> 16));
-    pixelOut[1] = (((pixelIn & 0x00ff00) >>  8));
-    pixelOut[2] = (((pixelIn & 0x0000ff) >>  0));
+  if (mBMPInfoHeader.bpp == 32) {
+    for (PRUint32 x = 0; x < aPixelWidth; x++) {
+      const PRUint32& pixelIn = ((const PRUint32*)(aSrc))[x];
+      PRUint8 *pixelOut = &aDest[x * bytes];
 
-    if (mBMPInfoHeader.bpp == 32) {
-      pixelOut[3] = alpha;
+      pixelOut[0] = (pixelIn & 0x00ff0000) >> 16;
+      pixelOut[1] = (pixelIn & 0x0000ff00) >>  8;
+      pixelOut[2] = (pixelIn & 0x000000ff) >>  0;
+      pixelOut[3] = (pixelIn & 0xff000000) >> 24;
+    }
+  } else {
+    for (PRUint32 x = 0; x < aPixelWidth; x++) {
+      const PRUint32& pixelIn = ((const PRUint32*)(aSrc))[x];
+      PRUint8 *pixelOut = &aDest[x * bytes];
+
+      pixelOut[0] = (pixelIn & 0xff0000) >> 16;
+      pixelOut[1] = (pixelIn & 0x00ff00) >>  8;
+      pixelOut[2] = (pixelIn & 0x0000ff) >>  0;
     }
   }
 }