diff --git a/gfx/ycbcr/yuv_convert.cpp b/gfx/ycbcr/yuv_convert.cpp
--- a/gfx/ycbcr/yuv_convert.cpp
+++ b/gfx/ycbcr/yuv_convert.cpp
@@ -36,19 +36,21 @@ NS_GFX_(void) ConvertYCbCrToRGB32(const 
                                   int pic_width,
                                   int pic_height,
                                   int y_pitch,
                                   int uv_pitch,
                                   int rgb_pitch,
                                   YUVType yuv_type) {
   unsigned int y_shift = yuv_type == YV12 ? 1 : 0;
   unsigned int x_shift = yuv_type == YV24 ? 0 : 1;
-  // There is no optimized YV24 MMX routine so we check for this and
+  // Test for SSE because the optimized code uses movntq, which is not part of MMX.
+  bool has_sse = supports_mmx() && supports_sse();
+  // There is no optimized YV24 SSE routine so we check for this and
   // fall back to the C code.
-  bool has_mmx = supports_mmx() && yuv_type != YV24;
+  has_sse &= yuv_type != YV24;
   bool odd_pic_x = yuv_type != YV24 && pic_x % 2 != 0;
   int x_width = odd_pic_x ? pic_width - 1 : pic_width;
 
   for (int y = pic_y; y < pic_height + pic_y; ++y) {
     uint8* rgb_row = rgb_buf + (y - pic_y) * rgb_pitch;
     const uint8* y_ptr = y_buf + y * y_pitch + pic_x;
     const uint8* u_ptr = u_buf + (y >> y_shift) * uv_pitch + (pic_x >> x_shift);
     const uint8* v_ptr = v_buf + (y >> y_shift) * uv_pitch + (pic_x >> x_shift);
@@ -60,32 +62,32 @@ NS_GFX_(void) ConvertYCbCrToRGB32(const 
                                  u_ptr++,
                                  v_ptr++,
                                  rgb_row,
                                  1,
                                  x_shift);
       rgb_row += 4;
     }
 
-    if (has_mmx)
+    if (has_sse)
       FastConvertYUVToRGB32Row(y_ptr,
                                u_ptr,
                                v_ptr,
                                rgb_row,
                                x_width);
     else
       FastConvertYUVToRGB32Row_C(y_ptr,
                                  u_ptr,
                                  v_ptr,
                                  rgb_row,
                                  x_width,
                                  x_shift);
   }
 
 #ifdef ARCH_CPU_X86_FAMILY
-  // MMX used for FastConvertYUVToRGB32Row requires emms instruction.
-  if (has_mmx)
+  // SSE used for FastConvertYUVToRGB32Row requires emms instruction.
+  if (has_sse)
     EMMS();
 #endif
 }
 
 }  // namespace gfx
 }  // namespace mozilla