--- a/dom/canvas/WebGLContextState.cpp
+++ b/dom/canvas/WebGLContextState.cpp
@@ -237,27 +237,25 @@ WebGLContext::GetParameter(JSContext* cx
             return JS::Int32Value(mImplMaxColorAttachments);
 
         } else if (pname == LOCAL_GL_MAX_DRAW_BUFFERS) {
             return JS::Int32Value(mImplMaxDrawBuffers);
 
         } else if (pname >= LOCAL_GL_DRAW_BUFFER0 &&
                    pname < GLenum(LOCAL_GL_DRAW_BUFFER0 + mImplMaxDrawBuffers))
         {
-            GLint iv = 0;
-            gl->fGetIntegerv(pname, &iv);
-
-            if (mBoundDrawFramebuffer)
-                return JS::Int32Value(iv);
-
-            const GLint index = (pname - LOCAL_GL_DRAW_BUFFER0);
-            if (iv == LOCAL_GL_COLOR_ATTACHMENT0 + index)
-                return JS::Int32Value(LOCAL_GL_BACK);
-
-            return JS::Int32Value(LOCAL_GL_NONE);
+            GLint ret = LOCAL_GL_NONE;
+            if (!mBoundDrawFramebuffer) {
+                if (pname == LOCAL_GL_DRAW_BUFFER0) {
+                    ret = gl->Screen()->GetDrawBufferMode();
+                }
+            } else {
+                gl->fGetIntegerv(pname, &ret);
+            }
+            return JS::Int32Value(ret);
         }
     }
 
     if (IsWebGL2() || IsExtensionEnabled(WebGLExtensionID::OES_vertex_array_object)) {
         if (pname == LOCAL_GL_VERTEX_ARRAY_BINDING) {
             WebGLVertexArray* vao =
                 (mBoundVertexArray != mDefaultVertexArray) ? mBoundVertexArray.get() : nullptr;
             return WebGLObjectAsJSValue(cx, vao, rv);

--- a/gfx/gl/GLScreenBuffer.h
+++ b/gfx/gl/GLScreenBuffer.h
@@ -236,19 +236,18 @@ public:
     void BeforeReadCall();
 
     bool CopyTexImage2D(GLenum target, GLint level, GLenum internalformat, GLint x,
                         GLint y, GLsizei width, GLsizei height, GLint border);
 
     void SetReadBuffer(GLenum userMode);
     void SetDrawBuffer(GLenum userMode);
 
-    GLenum GetReadBufferMode() const {
-        return mUserReadBufferMode;
-    }
+    GLenum GetReadBufferMode() const { return mUserReadBufferMode; }
+    GLenum GetDrawBufferMode() const { return mUserDrawBufferMode; }
 
     /**
      * Attempts to read pixels from the current bound framebuffer, if
      * it is backed by a SharedSurface.
      *
      * Returns true if the pixel data has been read back, false
      * otherwise.
      */