--- a/content/canvas/src/WebGLContext.h
+++ b/content/canvas/src/WebGLContext.h
@@ -2105,20 +2105,18 @@ public:
 };
 
 NS_DEFINE_STATIC_IID_ACCESSOR(WebGLExtension, WEBGLACTIVEINFO_PRIVATE_IID)
 
 /**
  ** Template implementations
  **/
 
-/* Helper function taking a BaseInterfaceType pointer and check that
- * it matches the required concrete implementation type (if it's
- * non-null), that it's not null/deleted unless we allowed it to, and
- * obtain a pointer to the concrete object.
+/* Helper function taking a BaseInterfaceType pointer, casting it to
+ * ConcreteObjectType and performing some checks along the way.
  *
  * By default, null (respectively: deleted) aInterface pointers are
  * not allowed, but if you pass a non-null isNull (respectively:
  * isDeleted) pointer, then they become allowed and the value at
  * isNull (respecively isDeleted) is overwritten.
  *
  * If generateErrors is true (which is the default) then upon errors,
  * GL errors are synthesized and error messages are printed, prepended by
@@ -2146,22 +2144,28 @@ WebGLContext::GetConcreteObject(const ch
                 ErrorInvalidValue("%s: null object passed as argument", info);
             return PR_FALSE;
         }
     }
 
     if (isNull)
         *isNull = PR_FALSE;
 
-    nsresult rv;
-    nsCOMPtr<ConcreteObjectType> tmp(do_QueryInterface(aInterface, &rv));
-    if (NS_FAILED(rv))
-        return PR_FALSE;
-
-    *aConcreteObject = tmp;
+#ifdef DEBUG
+    {
+        // once bug 694114 is implemented, we want to replace this by a static assertion, without #ifdef DEBUG
+        nsresult rv;
+        do_QueryInterface(aInterface, &rv);
+        NS_ABORT_IF_FALSE(NS_SUCCEEDED(rv),
+                          "QueryInterface failed. WebGL objects are builtinclass, so this should never happen. "
+                          "Please file a bug at bugzilla.mozilla.org -> Core -> Canvas:WebGL and link to the present page.");
+    }
+#endif
+    
+    *aConcreteObject = static_cast<ConcreteObjectType*>(aInterface);
 
     if (!(*aConcreteObject)->IsCompatibleWithContext(this)) {
         // the object doesn't belong to this WebGLContext
         if (generateErrors)
             ErrorInvalidOperation("%s: object from different WebGL context (or older generation of this one) "
                                   "passed as argument", info);
         return PR_FALSE;
     }

--- a/content/canvas/test/webgl/failing_tests_linux.txt
+++ b/content/canvas/test/webgl/failing_tests_linux.txt
@@ -12,10 +12,9 @@ conformance/gl-uniform-bool.html
 conformance/glsl-conformance.html
 conformance/glsl-long-variable-names.html
 conformance/premultiplyalpha-test.html
 conformance/read-pixels-test.html
 conformance/uninitialized-test.html
 conformance/more/conformance/quickCheckAPI.html
 conformance/more/functions/copyTexImage2D.html
 conformance/more/functions/copyTexSubImage2D.html
-conformance/more/functions/deleteBufferBadArgs.html
 conformance/more/functions/uniformfArrayLen1.html

--- a/content/canvas/test/webgl/failing_tests_mac.txt
+++ b/content/canvas/test/webgl/failing_tests_mac.txt
@@ -8,11 +8,10 @@ conformance/glsl-long-variable-names.htm
 conformance/premultiplyalpha-test.html
 conformance/program-test.html
 conformance/read-pixels-test.html
 conformance/tex-input-validation.html
 conformance/texture-npot.html
 conformance/more/conformance/quickCheckAPI.html
 conformance/more/functions/copyTexImage2D.html
 conformance/more/functions/copyTexSubImage2D.html
-conformance/more/functions/deleteBufferBadArgs.html
 conformance/more/functions/uniformfBadArgs.html
 conformance/more/functions/uniformiBadArgs.html

--- a/content/canvas/test/webgl/failing_tests_windows.txt
+++ b/content/canvas/test/webgl/failing_tests_windows.txt
@@ -3,10 +3,9 @@ conformance/drawingbuffer-test.html
 conformance/framebuffer-object-attachment.html
 conformance/glsl-conformance.html
 conformance/glsl-long-variable-names.html
 conformance/premultiplyalpha-test.html
 conformance/read-pixels-test.html
 conformance/more/conformance/quickCheckAPI.html
 conformance/more/functions/copyTexImage2D.html
 conformance/more/functions/copyTexSubImage2D.html
-conformance/more/functions/deleteBufferBadArgs.html
 conformance/more/functions/uniformfArrayLen1.html

--- a/dom/interfaces/canvas/nsIDOMWebGLRenderingContext.idl
+++ b/dom/interfaces/canvas/nsIDOMWebGLRenderingContext.idl
@@ -69,97 +69,97 @@ struct TypedArray;
 %}
 
 [ptr] native WebGLJSObjectPtr (JSObject);
 
 //
 // OpenGL object wrappers
 //
 
-[scriptable, uuid(0df9f4ed-f5ff-4e51-a6ff-2bd9785a7639)]
+[scriptable, builtinclass, uuid(0df9f4ed-f5ff-4e51-a6ff-2bd9785a7639)]
 interface nsIWebGLTexture : nsISupports
 {
   [noscript] attribute WebGLuint name;
 };
 
-[scriptable, uuid(9eca9c32-8305-11de-b89b-000c29206271)]
+[scriptable, builtinclass, uuid(9eca9c32-8305-11de-b89b-000c29206271)]
 interface nsIWebGLBuffer : nsISupports
 {
   [noscript] attribute WebGLuint name;
 };
 
-[scriptable, uuid(a6a19e74-8305-11de-9ce9-000c29206271)]
+[scriptable, builtinclass, uuid(a6a19e74-8305-11de-9ce9-000c29206271)]
 interface nsIWebGLProgram : nsISupports
 {
   [noscript] attribute WebGLuint name;
 };
 
-[scriptable, uuid(ac7440a4-8305-11de-807b-000c29206271)]
+[scriptable, builtinclass, uuid(ac7440a4-8305-11de-807b-000c29206271)]
 interface nsIWebGLShader : nsISupports
 {
   [noscript] attribute WebGLuint name;
 };
 
-[scriptable, uuid(beea4b38-3094-4e8d-b6e6-8b21d07e8994)]
+[scriptable, builtinclass, uuid(beea4b38-3094-4e8d-b6e6-8b21d07e8994)]
 interface nsIWebGLShaderArray : nsISupports
 {
     readonly attribute unsigned long length;
     nsIWebGLShader item(in unsigned long index);
 };
 
-[scriptable, uuid(bce8be60-8305-11de-9f3c-000c29206271)]
+[scriptable, builtinclass, uuid(bce8be60-8305-11de-9f3c-000c29206271)]
 interface nsIWebGLFramebuffer : nsISupports
 {
   [noscript] attribute WebGLuint name;
 };
 
-[scriptable, uuid(c82eacd0-8305-11de-9de9-000c29206271)]
+[scriptable, builtinclass, uuid(c82eacd0-8305-11de-9de9-000c29206271)]
 interface nsIWebGLRenderbuffer : nsISupports
 {
   [noscript] attribute WebGLuint name;
 };
 
-[scriptable, uuid(a85d4fd0-5b9f-4cb8-aeee-5a2c5c5bad76)]
+[scriptable, builtinclass, uuid(a85d4fd0-5b9f-4cb8-aeee-5a2c5c5bad76)]
 interface nsIWebGLActiveInfo : nsISupports
 {
     readonly attribute WebGLint size;
     readonly attribute WebGLenum type;
     readonly attribute DOMString name;
 };
 
-[scriptable, uuid(d38b0467-623e-4c82-9140-5f14a3bd1bad)]
+[scriptable, builtinclass, uuid(d38b0467-623e-4c82-9140-5f14a3bd1bad)]
 interface nsIWebGLUniformLocation : nsISupports
 {
     [noscript] attribute WebGLint location;
 };
 
 /* Base class for all WebGL extensions, and for those without
  * any additional tokens/methods.
  */
-[scriptable, uuid(70814f32-a5c5-4b95-83d5-ac653bac3316)]
+[scriptable, builtinclass, uuid(70814f32-a5c5-4b95-83d5-ac653bac3316)]
 interface nsIWebGLExtension : nsISupports
 {
 };
 
 /* Classes that extend on nsIWebGLExtension and provide
  * extra tokens, functions, etc.
  */
-[scriptable, uuid(3de3dfd9-864a-4e4c-989b-2977eaa80b7b)]
+[scriptable, builtinclass, uuid(3de3dfd9-864a-4e4c-989b-2977eaa80b7b)]
 interface nsIWebGLExtensionStandardDerivatives : nsIWebGLExtension
 {
   //
   //  CONSTANTS
   //
 
   /* Fragment shader hint */
   const WebGLenum FRAGMENT_SHADER_DERIVATIVE_HINT_OES   = 0x8B8B;
 };
 
 
-[scriptable, uuid(ef15ae85-4670-4dc4-848d-51ca81e8397a)]
+[scriptable, builtinclass, uuid(ef15ae85-4670-4dc4-848d-51ca81e8397a)]
 interface nsIDOMWebGLRenderingContext : nsISupports
 {
   //
   //  CONSTANTS
   //
 
   /* ClearBufferMask */
   const unsigned long DEPTH_BUFFER_BIT               = 0x00000100;