--- a/gfx/gl/GLContext.cpp
+++ b/gfx/gl/GLContext.cpp
@@ -134,16 +134,17 @@ static const char* const sExtensionNames
     "GL_EXT_draw_instanced",
     "GL_EXT_draw_range_elements",
     "GL_EXT_frag_depth",
     "GL_EXT_framebuffer_blit",
     "GL_EXT_framebuffer_multisample",
     "GL_EXT_framebuffer_object",
     "GL_EXT_framebuffer_sRGB",
     "GL_EXT_gpu_shader4",
+    "GL_EXT_map_buffer_range",
     "GL_EXT_multisampled_render_to_texture",
     "GL_EXT_occlusion_query_boolean",
     "GL_EXT_packed_depth_stencil",
     "GL_EXT_read_format_bgra",
     "GL_EXT_robustness",
     "GL_EXT_sRGB",
     "GL_EXT_sRGB_write_control",
     "GL_EXT_shader_texture_lod",

--- a/gfx/gl/GLContext.h
+++ b/gfx/gl/GLContext.h
@@ -435,16 +435,17 @@ public:
         EXT_draw_instanced,
         EXT_draw_range_elements,
         EXT_frag_depth,
         EXT_framebuffer_blit,
         EXT_framebuffer_multisample,
         EXT_framebuffer_object,
         EXT_framebuffer_sRGB,
         EXT_gpu_shader4,
+        EXT_map_buffer_range,
         EXT_multisampled_render_to_texture,
         EXT_occlusion_query_boolean,
         EXT_packed_depth_stencil,
         EXT_read_format_bgra,
         EXT_robustness,
         EXT_sRGB,
         EXT_sRGB_write_control,
         EXT_shader_texture_lod,

--- a/gfx/gl/GLContextFeatures.cpp
+++ b/gfx/gl/GLContextFeatures.cpp
@@ -367,16 +367,17 @@ static const FeatureInfo sFeatureInfoArr
         }
     },
     {
         "map_buffer_range",
         GLVersion::GL3,
         GLESVersion::ES3,
         GLContext::ARB_map_buffer_range,
         {
+            GLContext::EXT_map_buffer_range,
             GLContext::Extensions_End
         }
     },
     {
         "occlusion_query",
         GLVersion::GL2,
         GLESVersion::NONE,
         GLContext::Extension_None,

--- a/gfx/layers/opengl/CompositorOGL.cpp
+++ b/gfx/layers/opengl/CompositorOGL.cpp
@@ -121,32 +121,43 @@ AsyncReadbackBufferOGL::MapAndCopyInto(D
   MOZ_RELEASE_ASSERT(aReadSize <= aSurface->GetSize());
 
   if (!mGL || !mGL->MakeCurrent()) {
     return false;
   }
 
   ScopedPackState scopedPackState(mGL);
   Bind();
-  uint8_t* srcData = static_cast<uint8_t*>(
-    mGL->fMapBuffer(LOCAL_GL_PIXEL_PACK_BUFFER, LOCAL_GL_READ_ONLY));
+
+  const uint8_t *srcData = nullptr;
+  if (mGL->IsSupported(GLFeature::map_buffer_range)) {
+    srcData = static_cast<uint8_t*>(
+      mGL->fMapBufferRange(
+        LOCAL_GL_PIXEL_PACK_BUFFER,
+        0,
+        aReadSize.height * aReadSize.width * 4,
+        LOCAL_GL_MAP_READ_BIT));
+  } else {
+    srcData = static_cast<uint8_t*>(
+      mGL->fMapBuffer(LOCAL_GL_PIXEL_PACK_BUFFER, LOCAL_GL_READ_ONLY));
+  }
 
   if (!srcData) {
     return false;
   }
 
   int32_t srcStride = mSize.width * 4; // Bind() sets an alignment of 1
   DataSourceSurface::ScopedMap map(aSurface, DataSourceSurface::WRITE);
   uint8_t* destData = map.GetData();
   int32_t destStride = map.GetStride();
   SurfaceFormat destFormat = aSurface->GetFormat();
   for (int32_t destRow = 0; destRow < aReadSize.height; destRow++) {
     // Turn srcData upside down during the copy.
     int32_t srcRow = aReadSize.height - 1 - destRow;
-    uint8_t* src = &srcData[srcRow * srcStride];
+    const uint8_t* src = &srcData[srcRow * srcStride];
     uint8_t* dest = &destData[destRow * destStride];
     SwizzleData(src, srcStride, SurfaceFormat::R8G8B8A8,
                 dest, destStride, destFormat, IntSize(aReadSize.width, 1));
   }
 
   mGL->fUnmapBuffer(LOCAL_GL_PIXEL_PACK_BUFFER);
 
   return true;